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

[LilithHafner]

I copied the example in the README and got

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.

[Zentrik]

Per https://man7.org/linux/man-pages/man2/perf_event_open.2.html, the error is

Returned if an event requiring a specific hardware feature
is requested but there is no hardware support.  This
includes requesting low-skid events if not supported,
branch tracing if it is not available, sampling if no PMU
interrupt is available, and branch stacks for software
events.

It may be reasonable to add this as another error we specially handle at

LinuxPerf.jl/src/LinuxPerf.jl

Lines 301 to 305 in eae2705

	if errno in (Libc.EINVAL,Libc.ENOENT)
	if warn_unsupported
	@warn("$evt_type not supported, skipping")
	end
	continue

But supporting all the different errors seems like code bloat.