Cast errors generated during synapse_port_db to str (#8585)

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:

https://github.com/matrix-org/synapse/blob/d25eb8f3709965d0face01a041d5292490bf0139/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.