get_maintainer: quiet noisy implicit -f vcs_file_exists checking

Checking command line filenames that are outside the git tree can emit a
noisy and confusing message.

Quiet that message by redirecting stderr.
Verify that the command was executed successfully.

Fixes: 4cad35a7 ("get_maintainer.pl: reduce need for command-line option -f")
Link: http://lkml.kernel.org/r/1970a1d2fecb258e384e2e4fdaacdc9ccf3e30a4.1470955439.git.joe@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>