Running pipeline with unknown processor makes dpp fails silently

[vitorbaptista]

Python 3.6 @ Ubuntu 16.04

Consider the pipeline:

test:
  pipeline:
    -
      run: inexistent_task

When running it via dpp run:

$ dpp run ./test
INFO    :Main                            :Skipping redis connection, host:None, port:6379
INFO    :Main                            :RESULTS:

The same result happens if we try running an unknown pipeline, like:

$ dpp run ./this_doesnt_exist
INFO    :Main                            :Skipping redis connection, host:None, port:6379
INFO    :Main                            :RESULTS:

I would expect it to fail, ideally with a helpful error message (like we show when simply running dpp).