repo: Make locking precondition failures fatal
Use `g_error` and `g_assert*` rather than `g_return*` when checking the
locking preconditions so that failures result in the program
terminating. Since this code is protecting filesystem data, we'd rather
crash than delete or corrupt data unexpectedly.
`g_error` is used when the error is due to the caller requesting an
invalid transition like attempting to pop a lock type that hasn't been
taken. It also provides a semi-useful message about what happened.