restreamer: Don't 500 on bad timestamps in chat endpoint

restreamer/restreamer/main.py

@@ -458,8 +458,11 @@ def get_chat_messages(channel):
 		start, end: Required. The start and end times.
 			Must be in ISO 8601 format (ie. yyyy-mm-ddTHH:MM:SS) and UTC.
 	"""
+	try:
 		start = dateutil.parse_utc_only(request.args['start'])
 		end = dateutil.parse_utc_only(request.args['end'])
+	except ValueError:
+		return "Invalid timestamp", 400
 	if end <= start:
 		return "End must be after start", 400