git-am: print fair error message when format detection fails
git-am: print fair error message when format detection fails

Avoid git ending with this message:
	"Patch format  is not supported."

With improved error message in the format detection failure case by
Giuseppe Bilotta.

Signed-off-by: Nicolas Sebrecht <bfde528c540959b0444735ff13b324b56e999b57@laposte.net>
Signed-off-by: Brandon Casey <8846602a89ca53d44af0c0033a3c79dc5d05ea84@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git