perf tools: Emit clearer message for sys_perf_event_open ENOENT return

Resend of patch sent back in January 2011 in light of recent confusion around
unsupported events for a given platform.

Improve sys_perf_event_open ENOENT return handling in top and record, just
like 5a3446bc does for stat.

Retry of Arnaldo's patch using ui_warning instead of die which allows the
fallback from hardware cycles to software clock.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
LKML-Reference: <1301080271-20945-1-git-send-email-daahern@cisco.com>
Signed-off-by: David Ahern <daahern@cisco.com>
[ committer note: Some adjustments to make it apply to newer codebase ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>