perf help: Introduce exec_failed() to avoid code duplication

The warning(str_error_r(errno)) pattern can be replaced with a function,
do it.

And while at it use pr_warning(), we have way too many error reporting
facilities, time to drop some, starting with the one we got from the git
sources.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-lbak5npj1ri1uuvf1en3c0p0@git.kernel.org


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