rebase: make resolve message clearer for inexperienced users

rebase: make resolve message clearer for inexperienced users

The git UI can be improved by addressing the error messages to those
they help: inexperienced and casual git users. To this intent, it is
helpful to make sure the terms used in those messages can be understood
by this segment of users, and that they guide them to resolve the
problem.

In particular, failure to apply a patch during a git rebase is a common
problem that can be very destabilizing for the inexperienced user. It is
important to lead them toward the resolution of the conflict (which is a
3-steps process, thus complex) and reassure them that they can escape a
situation they can't handle with "--abort". This commit answer those two
points by detailling the resolution process and by avoiding cryptic git
linguo.

Signed-off-by: William Duclot <william.duclot@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>