$ python swarming.py trigger -s fed71d15123c6eda7afbb77319040f8b5ac70887  -S https://chromium-swarm.appspot.com -d os Ubuntu-12.04 -I https://isolateserver.appspot.com -T content_unittests

Sending the report ... done.
Report URL: https://chromium-swarm.appspot.com/restricted/ereporter2/errors/5864439062986752
Failed to trigger task content_unittests

---

Pretty much useless. The report is also useless.

Adding "-v" we see:

$ python swarming.py trigger -s fed71d15123c6eda7afbb77319040f8b5ac70887  -S https://chromium-swarm.appspot.com -d os Ubuntu-12.04 -I https://isolateserver.appspot.com -T content_unittests -v
....
1432 2017-02-08 20:36:44.573 I: Triggering: content_unittests
1432 2017-02-08 20:36:44.672 W: Able to connect to https://chromium-swarm.appspot.com/api/swarming/v1/tasks/new but an exception was thrown.
400 Client Error: Bad Request for url: https://chromium-swarm.appspot.com/api/swarming/v1/tasks/new
----------
Content-length: 83
Alt-svc: quic=":443"; ma=2592000; v="35,34"
Cache-control: no-cache
Server: Google Frontend
X-cloud-trace-context: a132a72684ed4ccf04c9a2d89f22acaa;o=5
Date: Wed, 08 Feb 2017 20:36:44 GMT
Access-control-allow-origin: *
Access-control-allow-methods: DELETE, GET, OPTIONS, POST, PUT
Content-type: application/json; charset=utf-8
Access-control-allow-headers: Origin, Authorization, Content-Type, Accept

{"error": {"message": "At least one of 'id' or 'pool' must be used as dimensions"}}
----------
Sending the report ...1432 2017-02-08 20:36:44.675 I: Starting new HTTPS connection (1): chromium-swarm.appspot.com
 done.
Report URL: https://chromium-swarm.appspot.com/restricted/ereporter2/errors/6021603727507456
Failed to trigger task content_unittests

------

So the error details are there, they are just ignored.