I noticed in https://github.com/matrix-org/synapse/issues/8575 that the `end_error` variable in `synapse_port_db` is set to an `Exception`, even though later we expect it to be a `str`.
This PR simply casts an exception raised to a string. I'm doing this instead of having `end_error` be of type exception as we explicitly set `end_error` to a str here:
d25eb8f370/scripts/synapse_port_db (L542-L547)
This whole file could probably use some heavy refactoring, but until then at least this fix will prevent exception contents from being hidden from us and users.
