sequencer: reschedule pick if index can't be locked

If the index cannot be locked in do_recursive_merge(), issue an error message and go on to the error recovery codepath, instead of dying. When the commit cannot be picked, it needs to be rescheduled when performing an interactive rebase, but just dying there won't allow that to happen, and when the user runs 'git rebase --continue' rather than 'git rebase --abort', the commit gets silently dropped. Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>