Recurring `MacaroonInitException: Must supply serialized macaroon.` in sentry

[DMRobertson]

Internal users: https://sentry.matrix.org/sentry/synapse-matrixorg/issues/219508/?query=is%3Aunresolved

First seen in 1.32.2, according to sentry.

Invalid session for OIDC callback

MacaroonInitException: Must supply serialized macaroon.
  File "synapse/handlers/oidc.py", line 220, in handle_oidc_callback
    session, state
  File "synapse/handlers/oidc.py", line 1186, in verify_oidc_session_token
    macaroon = pymacaroons.Macaroon.deserialize(session)
  File "pymacaroons/macaroon.py", line 50, in deserialize
    'Must supply serialized macaroon.'

Judging from the source code below, it looks like these are errors that we expect to encounter from time to time. Maybe logger.execption -> logger.warning?

synapse/synapse/handlers/oidc.py

Lines 217 to 229 in 6463244

	# Deserialize the session token and verify it.
	try:
	session_data = self._token_generator.verify_oidc_session_token(
	session, state
	)
	except (MacaroonInitException, MacaroonDeserializationException, KeyError) as e:
	logger.exception("Invalid session for OIDC callback")
	self._sso_handler.render_error(request, "invalid_session", str(e))
	return
	except MacaroonInvalidSignatureException as e:
	logger.exception("Could not verify session for OIDC callback")
	self._sso_handler.render_error(request, "mismatching_session", str(e))
	return

[DMRobertson]

Discussed this during triage. Consensus: this is sensible, let's do it.

[DMRobertson]

I raised #12782 with extra info from sandhose from the linked PR. I'd actually forgotten about this issue; I think it's best to close.