webhooks: Move UnexpectedWebhookEventType into zerver.lib.exceptions.

8e10ab282a moved UnexpectedWebhookEventType into
`zerver.lib.exceptions`, but left the import into
`zserver.lib.webhooks.common` so that webhooks could continue to
import the exception from there.

This clutters things and adds complexity; there is no compelling
reason that the exception's source of truth should not move alongside
all other exceptions.

zerver/lib/webhooks/common.py

@@ -10,12 +10,7 @@ from zerver.lib.actions import (
     check_send_stream_message,
     send_rate_limited_pm_notification_to_bot_owner,
 )
-from zerver.lib.exceptions import (
-    ErrorCode,
-    JsonableError,
-    StreamDoesNotExistError,
-    UnexpectedWebhookEventType,
-)
+from zerver.lib.exceptions import ErrorCode, JsonableError, StreamDoesNotExistError
 from zerver.lib.request import REQ, has_request_variables
 from zerver.lib.send_email import FromAddress
 from zerver.models import UserProfile