Butler should generate more helpful errors when pointed at any empty directory

>>>  Butler(tempfile.mkdtemp())

---------------------------------------------------------------------------

RuntimeError                              Traceback (most recent call last)

<ipython-input-5-4dc5ee5ae70e> in <module>()

----> 1 Butler(tempfile.mkdtemp())

/ssd/lsstsw/stack/Linux64/daf_persistence/12.1-17-g9654cba+1/python/lsst/daf/persistence/butler.pyc in __init__(self, root, mapper, inputs, outputs, **mapperArgs)

    302

    303         for args in outputs:

--> 304             self._addRepo(args, inout='out', defaultMapper=defaultMapper, butlerIOParents=butlerIOParents)

    305

    306         for args in inputs:

/ssd/lsstsw/stack/Linux64/daf_persistence/12.1-17-g9654cba+1/python/lsst/daf/persistence/butler.pyc in _addRepo(self, args, inout, defaultMapper, butlerIOParents, tags)

    397                     if defaultMapper is None:

    398                         raise RuntimeError(

--> 399                             "Could not infer mapper and one not specified in repositoryArgs:%s" % args)

    400                     args.mapper = defaultMapper

    401                 parents = [cfgRoot for cfgRoot in list(butlerIOParents.keys()) if cfgRoot != args.cfgRoot]

RuntimeError: Could not infer mapper and one not specified in repositoryArgs:RepositoryArgs(root='/tmp/tmp8pCn6I', cfgRoot=None, mapper=None, mapperArgs={}, tags=set([]), mode='rw', policy=None)