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>