Using hadoop fs -ls -d for testing file existence, instead of hadoop fs -test -e which is semi-broken

Added unit test to test connection errors which would previously have been seen as file non-existence
Improved exception messages from other hadoop cli calls

Change-Id: I5a0e37023777663cfdca74d95eeeb4f19b865b61
Reviewed-on: https://gerrit.spotify.net/gerrit/42362
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>

Conflicts:

	luigi/hdfs.py
New test that sets up a server in the background etc.

Trying to find a bug, but it's impossible. At least we have some new tests :)

Change-Id: I7a3fa8c1fb6716acb943a5dd533fcef81d8a7d57
Reviewed-on: https://gerrit.spotify.net/gerrit/42836
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>

Conflicts:

	debian/control
Fix file transactionality issue when using the with keyword on LocalTarget

Change-Id: Ib60adbf71bdc507104b9757b117f9f2a8f867d4f
Reviewed-on: https://gerrit.spotify.net/gerrit/44956
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>
Improved log messages for interface/worker execution

Change-Id: I0aa2670b66aeb6d795da9cb85466b4e838dae4ab
Reviewed-on: https://gerrit.spotify.net/gerrit/44958
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>