Better error message if requires() returns non-Tasks

Change-Id: Ieaf0f450dd3708772ffcd2f39700200058a3c301
Reviewed-on: https://gerrit.spotify.net/gerrit/43770
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>
Moved hdfs operations into HdfsClient class, added aliases so they can be used as module methods as before.

Change-Id: I972e45e595ab3af035d7c66034d40791b15b2b75
Reviewed-on: https://gerrit.spotify.net/gerrit/43945
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Anders Nyman <nyman@spotify.com>
Reviewed-by: Fabian Alenius <fabian@spotify.com>

Conflicts:

	luigi/hdfs.py