perf top: Add error message for EMFILE

When a user tries to open so many events, perf_event_open syscall may
fail with EMFILE. Provide advise for that case.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1325957132-10600-3-git-send-email-namhyung@gmail.com


Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>