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>