persistent storage is broken - "postgresql_stop() failed"

[Stuart Bishop]

Persistent storage attempts to shutdown PostgreSQL before migrating data. This fails because it is expecting an error code from postgresql_stop(), but postgresql_stop does not return anything (it raises an exception on failure).

        if not postgresql_stop():
            log("postgresql_stop() failed - can't migrate data.", ERROR)
            return False

Fix is to not check the error code, just catch or propagate the exception.

Keep the fix minimal, as this will conflict with the storage-subordinate-charm work