sequencer.c: check for lock failure and bail early in fast_forward_to

Change fast_forward_to() to check if locking the ref failed, print a
nice error message and bail out early.

The old code did not check if ref_lock was NULL and relied on the
fact that the write_ref_sha1() would safely detect this condition
and set the return variable ret to indicate an error.

While that is safe, it makes the code harder to read for two reasons:

 * Inconsistency.  Almost all other places we do check the lock for
   NULL explicitly, so the naive reader is confused "why don't we
   check here?"

 * And relying on write_ref_sha1() to detect and return an error for
   when a previous lock_any_ref_for_update() failed feels obfuscated.

This change should not change any functionality or logic aside from
adding an extra error message when this condition is triggered
(write_ref_sha1() returns an error silently for this condition).

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
commit.c: check for lock error and return early

Move the check for the lock failure to happen immediately after
lock_any_ref_for_update().  Previously the lock and the
check-if-lock-failed was separated by a handful of string
manipulation statements.

Moving the check to occur immediately after the failed lock makes
the code slightly easier to read and makes it follow the pattern of

 try-to-take-a-lock();
 if (check-if-lock-failed) {
    error();
 }

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'rs/ref-update-check-errors-early' into jch

* rs/ref-update-check-errors-early:
  commit.c: check for lock error and return early
  sequencer.c: check for lock failure and bail early in fast_forward_to