reftable/stack: use error codes when locking fails during compaction

Compaction of a reftable stack may fail gracefully when there is a
concurrent process that writes to the reftable stack and which has thus
locked either the "tables.list" file or one of the tables. This is
expected and can be handled gracefully by some of the callers which
invoke compaction. Thus, to indicate this situation to our callers, we
return a positive return code from `stack_compact_range()` and bubble it
up to the caller.

This kind of error handling is somewhat awkward though as many callers
in the call chain never even think of handling positive return values.
Thus, the result is either that such errors are swallowed by accident,
or that we abort operations with an unhelpful error message.

Make the code more robust by always using negative error codes when
compaction fails, with `REFTABLE_LOCK_ERROR` for the described benign
error case.

Note that only a single callsite knew to handle positive error codes
gracefully in the first place. Subsequent commits will touch up some of
the other sites to handle those errors better.

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

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