Make error_message() return the message

What is the expected behavior?

The error message will begin to come up from backends soon. IBMQ Provider needs to accommodate them.

The error message will be found in the qObjectResult in json as:

"qObjectResult": {
    <normal qObjectResult fields>
    "error": {
        "message": "Error message example",
        "code": 9999
    }
}

The error field will only appear if there is an error, so the job status should still be checked before attempting to access the "error".

This should be a simple enough matter of just digging a bit deeper into the job_response.

[jyu00]

I think the provider’s work on this is done for now. Just to do a quick recap, job.error_message() will retrieve the job result, and either

1. produce an error report if individual error message is available for each experiment
2. give a one-line error message if individual results are not available

Some parts of the stack might not be producing error messages right now, but when they do so in the correct format, the provider will automatically pick them up.

[nonhermitian]

This should be taken off hold as it is implemented on some devices now.