[Python] better error message on creating ParquetDataset from empty directory

>>> dataset = pq.ParquetDataset(path)
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-16-346f72ae525e> in <module>
----> 1 dataset = pq.ParquetDataset(path)

~/scipy/repos/arrow/python/pyarrow/parquet.py in __init__(self, path_or_paths, filesystem, schema, metadata, split_row_groups, validate_schema, filters, metadata_nthreads, memory_map)
    989 
    990         if validate_schema:
--> 991             self.validate_schemas()
    992 
    993         if filters is not None:

~/scipy/repos/arrow/python/pyarrow/parquet.py in validate_schemas(self)
   1025                 self.schema = self.common_metadata.schema
   1026             else:
-> 1027                 self.schema = self.pieces[0].get_metadata().schema
   1028         elif self.schema is None:
   1029             self.schema = self.metadata.schema

IndexError: list index out of range