Downgrade some OIDC exceptions to warnings (#12723)

Author: David Robertson

Diff for: changelog.d/12723.misc

@@ -0,0 +1 @@
+Downgrade some OIDC errors to warnings in the logs, to reduce the noise of Sentry reports.

Diff for: synapse/handlers/oidc.py

@@ -224,7 +224,7 @@ async def handle_oidc_callback(self, request: SynapseRequest) -> None:
             self._sso_handler.render_error(request, "invalid_session", str(e))
             return
         except MacaroonInvalidSignatureException as e:
-            logger.exception("Could not verify session for OIDC callback")
+            logger.warning("Could not verify session for OIDC callback: %s", e)
             self._sso_handler.render_error(request, "mismatching_session", str(e))
             return
 
@@ -827,7 +827,7 @@ async def handle_oidc_callback(
             logger.debug("Exchanging OAuth2 code for a token")
             token = await self._exchange_code(code)
         except OidcError as e:
-            logger.exception("Could not exchange OAuth2 code")
+            logger.warning("Could not exchange OAuth2 code: %s", e)
             self._sso_handler.render_error(request, e.error, e.error_description)
             return