No valid host nova error results in uninformative stack status

I hit this in TripleO CI - the root cause appears to be Nova returning 500 with a no-valid-host-found error, but Heat then obfuscates this pretty well:

What you see first as a user is:

2016-05-20 08:36:33.000 | | Controller | 26791d29-e7d0-45dd-ae04-02f99722d0b5 | ResourceInError: resources.Controller.resources[0].resources.Controller: Went to status ERROR due to "Message: Unknown, Code: Unknown" | CREATE_FAILED | 2016-05-20T08:32:49 |

So, a ResourceGroup, containing nested stacks (that contain servers) failed to create, and the reason is "Unknown, Code: Unknown", great! :(

There's no indication what the reason is, until you look in the logs:

2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource Traceback (most recent call last):
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 715, in _action_recorder
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource yield
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 795, in _do_action
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource yield self.action_handler_task(action, args=handler_args)
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/scheduler.py", line 329, in wrapper
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource step = next(subtask)
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 763, in action_handler_task
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource done = check(handler_data)
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resources/openstack/nova/server.py", line 903, in check_create_complete
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource check = self.client_plugin()._check_active(server_id)
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/clients/os/nova.py", line 223, in _check_active
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource 'code': fault.get('code', _('Unknown'))
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource ResourceInError: Went to status ERROR due to "Message: No valid host was found. There are not enough hosts available., Code: 500"
2016-05-20 08:32:45.692 20437 ERROR heat.engine.resource

Nova has given us 500, but it did give us a reasonable message.

This causes the stack resource to fail, at which point all useful information about why the error happened is lost:

2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource Traceback (most recent call last):
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 715, in _action_recorder
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource yield
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 795, in _do_action
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource yield self.action_handler_task(action, args=handler_args)
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/scheduler.py", line 329, in wrapper
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource step = next(subtask)
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resource.py", line 763, in action_handler_task
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource done = check(handler_data)
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resources/stack_resource.py", line 365, in check_create_complete
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource return self._check_status_complete(self.CREATE)
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource File "/usr/lib/python2.7/site-packages/heat/engine/resources/stack_resource.py", line 414, in _check_status_complete
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource action=action)
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource ResourceFailure: ResourceInError: resources[0].resources.Controller: Went to status ERROR due to "Message: Unknown, Code: Unknown"
2016-05-20 08:32:47.844 20439 ERROR heat.engine.resource

So, I guess the question is how do we surface deeply nested errors in a way that's more useful than "unknown"?