Can't print errors during tests

This makes debugging quite infuriating ;)

```
test DArray reduce: Error During Test at /home/vchuravy/.julia/dev/DistributedArrays/test/darray.jl:217
Got exception outside of a @test
scalar indexing disabled
Stacktrace:
[1] _scalarindexingallowed at /home/vchuravy/.julia/dev/DistributedArrays/src/darray.jl:582 [inlined]
[2] getindex at /home/vchuravy/.julia/dev/DistributedArrays/src/darray.jl:598 [inlined]
[3] _show_nonempty(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::DArray{Int64,2,Array{Int64,2}}, ::String) at ./arrayshow.jl:386
[4] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::DArray{Int64,2,Array{Int64,2}}) at ./arrayshow.jl:421
[5] show_delim_array(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DArray{Int64,2,Array{Int64,2}}}, ::Char, ::Char, ::Char, ::Bool, ::Int64, ::Int64) at ./show.jl:695
[6] show_delim_array at ./show.jl:680 [inlined]
[7] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Tuple{DArray{Int64,2,Array{Int64,2}}}) at ./show.jl:714
[8] show_default(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:332
[9] show(::IOContext{Base.GenericIOBuffer{Array{UInt8,1}}}, ::Any) at ./show.jl:315
[10] #sprint#325(::Pair{Symbol,Bool}, ::Int64, ::Function, ::Function, ::MethodError) at ./strings/io.jl:99
[11] #sprint at ./none:0 [inlined]
[12] Test.Error(::Symbol, ::Expr, ::MethodError, ::Array{Union{Ptr{Nothing}, InterpreterIP},1}, ::LineNumberNode) at /build/julia/src/julia/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:144
[13] top-level scope at /build/julia/src/julia/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1090
[14] include(::String) at ./client.jl:388
[15] top-level scope at none:0
[16] include(::String) at ./client.jl:388
[17] top-level scope at none:0
```

Start with the failing test at test/darray.jl:217 and trace the reported calls through src/darray.jl:582 and src/darray.jl:598. Compare the failure output with Julia's Test handling in Test.jl:144 and Test.jl:1090; done means test failures can print the relevant error without triggering the shown secondary exception.

Scritto dal modello di indicizzazione a partire dal testo della issue.