Cast errors generated during synapse_port_db to str (matrix-org#8585)

I noticed in matrix-org#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.

changelog.d/8585.bugfix

@@ -0,0 +1 @@
+Fix a bug that prevented errors encountered during execution of the `synapse_port_db` from being correctly printed.

scripts/synapse_port_db

@@ -22,6 +22,7 @@ import logging
 import sys
 import time
 import traceback
+from typing import Optional
 
 import yaml
 
@@ -152,7 +153,7 @@ IGNORED_TABLES = {
 
 # Error returned by the run function. Used at the top-level part of the script to
 # handle errors and return codes.
-end_error = None
+end_error = None  # type: Optional[str]
 # The exec_info for the error, if any. If error is defined but not exec_info the script
 # will show only the error message without the stacktrace, if exec_info is defined but
 # not the error then the script will show nothing outside of what's printed in the run
@@ -635,7 +636,7 @@ class Porter(object):
             self.progress.done()
         except Exception as e:
             global end_error_exec_info
-            end_error = e
+            end_error = str(e)
             end_error_exec_info = sys.exc_info()
             logger.exception("")
         finally: