Improve the error message for when a new org name doesn't pass the regex.

data/model.py

@@ -111,7 +111,10 @@ def create_organization(name, email, creating_user):
 
     return new_org
   except InvalidUsernameException:
-    raise InvalidOrganizationException('Invalid organization name: %s' % name)
+    msg = ('Invalid organization name: %s Organization names must consist ' +
+           'solely of lower case letters, numbers, and underscores. ' + 
+           '[a-z0-9_]') % name
+    raise InvalidOrganizationException(msg)
 
 
 def create_robot(robot_shortname, parent):