perf tools: Notify user when unrecognized event is specified

Previously no indication was given about what went wrong.

Signed-off-by: Marti Raudsepp <marti@juffo.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <03ec9ee96f17cef05424.1256603584@localhost>
Signed-off-by: Ingo Molnar <mingo@elte.hu>