Make missing managers.xml result in a clear error message/exception

If managers.xml cannot be found as a ClassLoader resource, the following is thrown:

org.jboss.xb.binding.JBossXBException: Failed to parse source: null
at org.jboss.xb.binding.parser.sax.SaxJBossXBParser.parse(SaxJBossXBParser.java:203)
at org.jboss.xb.binding.parser.sax.SaxJBossXBParser.parse(SaxJBossXBParser.java:183)
at org.jboss.xb.binding.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:190)
at org.jboss.jmx.adaptor.snmp.agent.TrapEmitter.load(TrapEmitter.java:243)
at org.jboss.jmx.adaptor.snmp.agent.TrapEmitter.start(TrapEmitter.java:105)

First check that the resource can be found, and throw an appropriate Exception if not.