Mongo decode result (fixes #2458)

[buckensl]

Decode result and convert serialized exception to Python exception
fixes #2458

[coveralls]

Coverage decreased (-0.01%) to 90.26% when pulling 91948ef on buckensl:mongo-result-decode into db2caf5 on celery:master.

[thedrow]

Sorry for the late response.
This needs a test that verifies that the fix works.

[ask]

I don't get it, why would obj["result"] contain yet another level of meta["status"], etc?

The structure is usually:
{'status': 'FAILURE', 'result': exception}

not:
{'status': 'FAILURE', 'result': {'status': FAILURE, 'result': exception}}

[ask]

This has been open for too long without a resolution :( We need to be faster at merging these in the future, but need to make sure all pull requests have tests first.