perf target: Introduce perf_target_errno

The perf_target_errno enumerations are used to indicate specific error
cases on perf target operations. It'd help libperf being a more generic
library.

Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1336367344-28071-4-git-send-email-namhyung.kim@lge.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>