Improve DTest package API and error messages

The DTest Instrumentor currently requires a class to be presented in the dtest JVM API when scheduling instrumentation of calls to methods of that class in the remote traced JVM. It uses reflection to filter the supplied list of method names, rejecting those whcih do not match a method of the class. This has the unfortunate side effect of requiring the dtest JVM to have access to all the jars visible in the traced JVM, which can be inconvenient – especially when the latter is a JBoss EAP instance whose jars are bundled somewhere down in the EAP module directory tree. This request is for an alternative version of the API which simply uses a class name and list of method names to generate trace rules without requiring a class against which to perform the reflective validation.

The request also bundles a few small changes ot the message API to improve usability and readability.