Document what the "ERROR: perf_event_open error : Operation not supported" error means

I copied the example in the README and got
```julia-repl
ulia> data = zeros(10000); @measure g(data)
ERROR: perf_event_open error : Operation not supported
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:35
[2] LinuxPerf.EventGroup(types::Vector{LinuxPerf.EventType}; warn_unsupported::Bool, userspace_only::Bool, pinned::Bool, exclusive::Bool, pid::Int32)
@ LinuxPerf ~/.julia/packages/LinuxPerf/396gD/src/LinuxPerf.jl:312
[3] EventGroup
@ ~/.julia/packages/LinuxPerf/396gD/src/LinuxPerf.jl:240 [inlined]
[4] make_bench(x::Vector{Any})
@ LinuxPerf ~/.julia/packages/LinuxPerf/396gD/src/LinuxPerf.jl:476
[5] top-level scope
@ ~/.julia/packages/LinuxPerf/396gD/src/LinuxPerf.jl:22
```
I'm guessing this means my system doesn't support these perf counters. It'd be nice to print a more helpful error message if so.