perf stat: Error out unsupported group leader immediately

perf stat ignores the unsupported event and continue to count supported
event. But if the unsupported event is group leader, perf tool will
crash. After applying this patch, the unsupported group leader will
error out immediately.

Without this patch:

  $ perf stat -x, -e '{node-prefetch-refs,cycles}' --  sleep 1
  perf: util/evsel.c:1009: get_group_fd: Assertion `!(fd == -1)' failed.
  Aborted (core dumped)

With this patch:

  $ perf stat -x, -e '{node-prefetch-refs,cycles}' --  sleep 1
  Error:
  The node-prefetch-refs event is not supported.

Commiter note: Here I got a different output, but no core dump:

  [acme@zoo linux]$ perf stat -x, -e '{node-prefetch-refs,cycles}' -- sleep 1
  Error:
  The sys_perf_event_open() syscall returned with 22 (Invalid argument)
  for event (node-prefetch-refs).
  /bin/dmesg may provide additional information.
  No CONFIG_PERF_EVENTS=y kernel support configured?

Signed-off-by: Kan Liang <kan.liang@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Link: http://lkml.kernel.org/r/1434004360-8570-1-git-send-email-kan.liang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>