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 <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-7y40wib8n006io7hjpn1dsrm@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@elte.hu>