perf stat: Factor out open error handling

Factor out the open error handling into a separate function.  This is
useful for followon patches who need to duplicate this.

No behavior change intended.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20191121001522.180827-9-andi@firstfloor.org


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