cherry-pick/revert: Use error() for failure message
cherry-pick/revert: Use error() for failure message

When cherry-pick fails after picking a large series of commits, it can
be hard to pick out the error message and advice.  Clarify the error
and prefix it with “error: ” to help.

Before:

	Automatic cherry-pick failed.  [...advice...]

After:

	error: could not apply 7ab78c9... Do something neat.
	[...advice...]

Noticed-by: Thomas Rast <18f23ef7096473d837b7404952f13710423b8b63@student.ethz.ch>
Encouraged-by: Sverre Rabbelier <91e3618f486cb75969cf54d52cb06350cfe172f2@gmail.com>
Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@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