Added validation for fcr:import; improved error responses.

Please, @mikedurbin, tell me there is a way to get at the functionality in JCRXMLValidatingInputStreamBridge and XMLReaderBridge by leveraging some libraries instead of what amounts to writing C code. The bugginess and maintainability here is a bit frightening.
Can you not do something with XMLInputFactory.createXMLStreamReader or .createXMLEventReader that does not require all of the pass-through methods below and buffer logic?
There must be a library for inspecting the reading of an inputstream for XML events. commons-io? guava? javax.xml? Something... please, something.