Description of problem:
Since the spacewalk-java-2.5.14-103, after scheduling an event with child events (e.g. provisioning a system) it is not possible to cancel a child event using the WebUI. It is still possible to try cancelling it using the API, leading to "redstone.xmlrpc.XmlRpcFault: unhandled internal exception: null". As it is not possible to restrict types of events API can try to cancel, it would be useful to return a more useful error message instead. 

Version-Release number of selected component (if applicable):
spacewalk-java-2.5.14-105

How reproducible:
always

Steps to Reproduce:
1. Schedule provisioning for a system
2. Get the event id for the child event (System reboot for example)
3. Try running client.schedule.cancelActions(key, <child_event_id>)

Actual results:
client.schedule.cancelActions(key, [31])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1199, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1489, in __request
    verbose=self.__verbose
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1253, in request
    return self._parse_response(h.getfile(), sock)
  File "/usr/lib64/python2.6/xmlrpclib.py", line 1392, in _parse_response
    return u.close()
  File "/usr/lib64/python2.6/xmlrpclib.py", line 838, in close
    raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault -1: 'redstone.xmlrpc.XmlRpcFault: unhandled internal exception: null'>

Expected results:
A more descriptive error message.

Additional info:
For more info regarding the WebUI version of this bug, see BZ1492572.