Use the correct CL to load notifications.xml

Embedded in a full runmode has the notifications.xml config packaged separately from (hence loaded by a different CL than) TrapFactorySupport.class. Resultant error is:

2009-11-03 11:34:23,438 ERROR [org.jboss.jmx.adaptor.snmp.agent.TrapFactorySupport] (Thread-1) Accessing resource '/notifications.xml'
2009-11-03 11:34:23,439 ERROR [org.jboss.kernel.plugins.dependency.AbstractKernelController] (Thread-1) Error installing to Start: name=jboss.jmx:name=SnmpAgent,service=snmp,type=adaptor state=Create mode=Manual requiredState=Installed
org.jboss.xb.binding.JBossXBException: Failed to parse source: null

Also, make a better error message when not found (don't allow to propagate down into JBossXB, throw right in place)