If the connection to the backend where a read-only transaction is being
performed fails, the Backend object should be closed for it. This fixes a
debug assertion in readwritesplit.cc:check_and_log_backend_state which
asserts that the failed connection must not be in use after the error
handling is done.
Also reordered the failing assertion and the accompanying error message so
that the error is logged first.
16f201beed