Exception specification for Job.result()

[yaelbh]

Job.result() is defined as https://github.com/Qiskit/qiskit-terra/blob/19fdd32affd37afb58774f40bcf509b16480fe63/qiskit/providers/job.py#L130-L133

We've noticed that IBMQJob.result() raises an exception for jobs that failed when they were executed, whereas AerJob.result() does not fail but returns a valid value (https://github.com/Qiskit/qiskit-aer/issues/1567).

In my opinion, types of returned values and exception policy should be uniform for all. As such, they are to be defined at the level of the base class.