Fix deprecation warning

We should use raise_if_exists over fail_if_exists. I guess my clever and
tricky `deprecate_kwarg` method is a bit too meta. :)
_hdfs_test: Fix failing test case

I think this started to fail since the changes I introduced in
spotify/luigi#393.
Merge pull request #456 from Tarrasch/fix-_hdfs_test

Fix hdfs tests