reftable/error: discern locked/outdated errors

reftable/error: discern locked/outdated errors

We currently throw two different errors into a similar-but-different
error code:

  - Errors when trying to lock the reftable stack.

  - Errors when trying to write to the reftable stack which has been
    modified concurrently.

This results in unclear error handling and user-visible error messages.

Create a new `REFTABLE_OUTDATED_ERROR` so that those error conditions
can be clearly told apart from each other. Adjust users of the old
`REFTABLE_LOCK_ERROR` to use the new error code as required.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>