Bug 1291070 - Return process exit code properly; r=glandium

Before, we were returning None, which gets converted to 0.
Derp.

Also fix a flake8 failure introduced by 9f5fbb3066c9.

We'll also need to generate a new decision image. But that will
require someone with TC privileges to be around. That can be done
in a separate commit to unblock this from landing and fixing
consumers of run-task that aren't the decision image.

MozReview-Commit-ID: 6XuoIxjDozF

--HG--
extra : rebase_source : 836f4996cb9be24e19504f73792b159f0ffbdbee

taskcluster/taskgraph/task/docker_image.py

@@ -73,8 +73,9 @@ class DockerImageTask(base.Task):
             image_artifact_path = \
                 "public/docker_image_contexts/{}/context.tar.gz".format(image_name)
             if os.environ.get('TASK_ID'):
-                # We put image context tar balls in a different artifacts folder on the
-                # Gecko decision task in order to have longer expiration dates for smaller artifacts
+                # We put image context tar balls in a different artifacts folder
+                # on the Gecko decision task in order to have longer expiration
+                # dates for smaller artifacts.
                 destination = os.path.join(
                     os.environ['HOME'],
                     "docker_image_contexts/{}/context.tar.gz".format(image_name))

testing/docker/recipes/run-task

@@ -61,7 +61,7 @@ def run_and_prefix_output(prefix, args):
 
         print_line(prefix, data)
 
-    return p.returncode
+    return p.wait()
 
 
 def vcs_checkout(args):