synapse_port_db error handling is broken.

[erikjohnston]

When there is an error, e.g. like in #7654, we fail to handle it properly and bail with a separate error:

Traceback (most recent call last):
  File "/root/synapse/env/bin/synapse_port_db", line 1056, in <module>
    sys.stderr.write(end_error)
TypeError: write() argument must be str, not FirstError

[richvdh]

fixed by #8585 I think