Revision 776412

o first pass at setting up error handling for each possible error condition
 - this resulted in cleaning up some exceptions in plexus so that they might message could be surfaced in Maven
o move the plugin prefix searching to the lifecycle executor and get it out of the plugin manager
o add capability to the RepositorySystem to get plain resources from a repository. I'm going simple with the repository group
  metadata and I will just get the file directly as the only implementation is the plugin group metadata. the system is 
  not easily or generally extensible so i'll just take what I need