implement error message in job

[korgan00]

Summary

If job.status() is ERROR, return raw result in error_message() and raise an exception in result()
else return an empty string in error_message() and return raw result in result()

Details and comments

[Tansito]

LGTM 🎉

[Tansito]

@pandasa123 we should update documentation with this right, where should we take a look?