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 <trast@student.ethz.ch>
Encouraged-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>