revert: improve error message for cherry-pick during cherry-pick

revert: improve error message for cherry-pick during cherry-pick

In the spirit of v1.6.3.3~3^2 (refuse to merge during a merge,
2009-07-01), "git cherry-pick" refuses to start a new cherry-pick when
in the middle of an existing conflicted cherry-pick in the following
sequence:

 1. git cherry-pick HEAD..origin
 2. resolve conflicts
 3. git cherry-pick HEAD..origin (instead of "git cherry-pick
    --continue", by mistake)

Good.  However, the error message on attempting step 3 is more
convoluted than necessary:

  $ git cherry-pick HEAD..origin
  error: .git/sequencer already exists.
  error: A cherry-pick or revert is in progress.
  hint: Use --continue to continue the operation
  hint: or --quit to forget about it
  fatal: cherry-pick failed

Clarify by removing the redundant first "error:" message, simplifying
the advice, and using lower-case and no full stops to be consistent
with other commands that prefix their messages with "error:", so it
becomes

  error: a cherry-pick or revert is already in progress
  hint: try "git cherry-pick (--continue | --quit)"
  fatal: cherry-pick failed

The "fatal: cherry-pick failed" line seems unnecessary, too, but
that can be fixed some other day.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>