DAX API should tell which lfn was a dup

The Python DAX API (and maybe the others have similar problems) currently just throws a generic DuplicateError in case a lfn is added twice:

  File "/usr/lib64/python2.7/site-packages/Pegasus/DAX3.py", line 929, in addUse
    raise DuplicateError("Duplicate Use", use)

But it does not tell what the lfn was so it can be difficult to debug. Similarly, the in-DAX rc thows a non specific error if you do the same addFile twice.

The errors should be updated to include the lfn.