xrootd4j: Add getters to ErrorResponse

To allow plugins to intercept and translate errors, the error number
and message is made available through getters.

Target: trunk, 1.3
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6821/
Merge branch 'fix/error-response-getters'