Need a way to load classes without eating exceptions

[ctrueden]

@EikeHeinz encountered a problem where ClassUtils.loadClass returns null, eating the exception as designed. But then the calling code in PluginInfo.loadClass throws another exception (InstantiableException) with the message "Class not found" with no cause.

We probably want ClassUtils.loadClass signatures that throw some unified exception (perhaps IllegalArgumentException) which can be called by code that needs to know why the class loading fails. A lot of code doesn't, but in this case, the PluginInfo.loadClass would benefit from knowing.

Just need to be careful about naming.