vinyl: fail aborted transactions early

We abort transactions when switching to read-only mode, building a new
index, or reverting a statement after a failed WAL write. It's no use
to allow an aborted transaction to proceed as usual until commit - we
should fail it as early as we can to avoid wasting resources. Currently,
we do it in a rather abrupt way - by an assertion :-) This patch makes
any DML/DQL operation fail gracefully instead.

Closes #4070