Inform backend on JSON parse error

[joepvd]

If you specify the following in your .travis.yml:

osx_image: 9.1

Worker will crap out with:

level=error
msg="start attributes JSON parse error, attempting to ack+drop delivery"
err="json: cannot unmarshal number into Go struct field StartAttributes.osx_image of type string" 

There are two problems here:

1. Arguably, this field should be cast into a string
2. The error handling should improve, as hub has not been informed of the inability to run the job when this error occured. After hub's timeout, the job was marked as done, and in that time, this defunct job consumed a slot of the concurrency.

Reference: https://admin.travis-ci.org/job/311441913