perf stat: better error message for unsupported events

For unsupported events (e.g., H/W events when running in a VM)
perf stat currently fails with the error message:

      Error: open_counter returned with 2 (No such file or directory).
    /bin/dmesg may provide additional information.

      Fatal: Not all events could be opened.

dmesg is of no help and it is not clear as to why it fails to
open the counter. This patch changes the error message to

      Error: cache-misses event is not supported.
      Fatal: Not all events could be opened.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: a.p.zijlstra@chello.nl
LPU-Reference: <1294597272-17335-1-git-send-email-daahern@cisco.com>
Signed-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>