Require some update in updateSuperevent

Require that some parameter is set to be upgraded for
updateSuperevent(), otherwise fail.

ligo/gracedb/rest.py

@@ -742,6 +742,10 @@ class GraceDb(GsiRest):
             >>> r.status
             200
         """
+        # Make sure that at least one parameter is provided
+        if not (t_start or t_0 or t_end or preferred_event):
+            raise ValueError('Provide at least one of t_start, t_0, t_end, or '
+                'preferred_event')
 
         request_body = {}
         if t_start: