perf stat: Fail softly on unsupported events

perf stat: Fail softly on unsupported events

David Ahern reported this perf stat failure:

> # /tmp/build-perf/perf stat -- sleep 1
>   Error: stalled-cycles-frontend event is not supported.
>   Fatal: Not all events could be opened.
>
> This is a Dell R410 with an E5620 processor.

Fail in a softer fashion on unknown/unsupported events.

Reported-by: David Ahern <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>