BigQuery _EnumProperty ValueError messages are not displayed properly (googleapis#3520)

Author: smasue

bigquery/google/cloud/bigquery/_helpers.py

@@ -318,7 +318,7 @@ def _validate(self, value):
         :raises: ValueError if value is not allowed.
         """
         if value not in self.ALLOWED:
-            raise ValueError('Pass one of: %s' ', '.join(self.ALLOWED))
+            raise ValueError('Pass one of: %s' % ', '.join(self.ALLOWED))
 
 
 class UDFResource(object):