Bug 1573375 - Wrap fetch-content output in run-task. r=tomprince

Like the output from other subcommands.

Differential Revision: https://phabricator.services.mozilla.com/D41706

taskcluster/scripts/run-task

@@ -512,8 +512,10 @@ def fetch_artifacts():
         sys.exit(1)
 
     cmd = [sys.executable, '-u', fetch_content, 'task-artifacts']
-    print_line(b'fetches', b'executing %r\n' % cmd)
-    subprocess.run(cmd, check=True, env=os.environ)
+    res = run_and_prefix_output(b'fetches', cmd)
+    if res:
+        sys.exit(res)
+
     print_line(b'fetches', b'finished fetching artifacts\n')