Catch Pkg errors and display a nicer error instead

When a student tries to load a package through `Pkg.add`, the error is rather cryptic. We could probably catch that and display a nicer error instead.

Or overload `Pkg` methods with a simple error message like `Pkg operations are not supported by the test runner`.

```julia
LoadError: LoadError: failed to clone from https://github.com/JuliaArrays/OffsetArrays.jl, error: GitError(Code:ERROR, Class:Net, failed to resolve address for github.com: Temporary failure in name resolution)
Stacktrace:
[1] pkgerror(msg::String)
@ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:55
[2] clone(ctx::Pkg.Types.Context, url::String, source_path::String; header::Nothing, credentials::Nothing, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:isbare,), Tuple{Bool}}})
@ Pkg.GitTools [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:119
[3] #ensure_clone#2
@ [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:72 [inlined]
[4] handle_repo_add!(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
@ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:619
[5] handle_repos_add!(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec})
@ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:686
[6] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:177
[7] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:148 [inlined]
[8] add(pkgs::Vector{Pkg.Types.PackageSpec}; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:73
[9] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
[10] #add#20
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
[11] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
[12] add(; name::Nothing, uuid::Nothing, version::Nothing, url::String, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:91
[13] top-level scope
@ ./pangram.jl:2
[14] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[15] include(x::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1
[16] top-level scope
@ ./runtests.jl:3
[17] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[18] include
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1 [inlined]
[19] macro expansion
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
[20] macro expansion
@ [...]/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
[21] macro expansion
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
[22] macro expansion
@ ~/.julia/packages/Suppressor/nTjgZ/src/Suppressor.jl:127 [inlined]
[23] runtests(testfile::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:13
[24] test_runner(exercise_slug::String, solution_dir::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:18
[25] test_runner(exercise_slug::String, solution_dir::String, output_dir::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:39
[26] top-level scope
@ /opt/test-runner/run.jl:3
[27] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[28] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:285
[29] _start()
@ Base ./client.jl:485
in expression starting at ./pangram.jl:2
in expression starting at ./runtests.jl:3

caused by: GitError(Code:ERROR, Class:Net, failed to resolve address for github.com: Temporary failure in name resolution)
Stacktrace:
[1] macro expansion
@ [...]/julia/stdlib/v1.6/LibGit2/src/error.jl:110 [inlined]
[2] clone(repo_url::SubString{String}, repo_path::String, clone_opts::LibGit2.CloneOptions)
@ LibGit2 [...]/julia/stdlib/v1.6/LibGit2/src/repository.jl:459
[3] clone(repo_url::SubString{String}, repo_path::String; branch::String, isbare::Bool, remote_cb::Ptr{Nothing}, credentials::LibGit2.CachedCredentials, callbacks::Dict{Symbol, Tuple{Ptr{Nothing}, Any}})
@ LibGit2 [...]/julia/stdlib/v1.6/LibGit2/src/LibGit2.jl:583
[4] clone(ctx::Pkg.Types.Context, url::String, source_path::String; header::Nothing, credentials::Nothing, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:isbare,), Tuple{Bool}}})
@ Pkg.GitTools [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:109
[5] #ensure_clone#2
@ [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:72 [inlined]
[6] handle_repo_add!(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
@ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:619
[7] handle_repos_add!(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec})
@ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:686
[8] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:177
[9] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:148 [inlined]
[10] add(pkgs::Vector{Pkg.Types.PackageSpec}; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:73
[11] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
[12] #add#20
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
[13] add
@ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
[14] add(; name::Nothing, uuid::Nothing, version::Nothing, url::String, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
@ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:91
[15] top-level scope
@ ./pangram.jl:2
[16] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[17] include(x::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1
[18] top-level scope
@ ./runtests.jl:3
[19] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[20] include
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1 [inlined]
[21] macro expansion
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
[22] macro expansion
@ [...]/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
[23] macro expansion
@ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
[24] macro expansion
@ ~/.julia/packages/Suppressor/nTjgZ/src/Suppressor.jl:127 [inlined]
[25] runtests(testfile::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:13
[26] test_runner(exercise_slug::String, solution_dir::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:18
[27] test_runner(exercise_slug::String, solution_dir::String, output_dir::String)
@ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:39
[28] top-level scope
@ /opt/test-runner/run.jl:3
[29] include(mod::Module, _path::String)
@ Base ./Base.jl:386
[30] exec_options(opts::Base.JLOptions)
@ Base ./client.jl:285
[31] _start()
@ Base ./client.jl:485
```