Handling maintenance responses

[francabrera]

What is the expected feature or enhancement?

When the IBM Quantum channel platform is on maintenance, request to the APIs will respond with an HTTP code 503 and a similar body to this:

{
  "error": {
    "statusCode": 503,
    "name": "MAINTENANCE",
    "message": "Planned maintenance. Please check https://docs.quantum.ibm.com/announcements/service-alerts",
    "code": 1999
  }
}

The provider should be able to handle this response and display the message in the body.

This was attempted to be fixed previously in #1647, however the solution would have not worked, because:

• the fix was under the assumption that the auth.quantum-computing.ibm.com/api would still be responding, which was a bug on the maintenance process of the service not putting this also in maintenance (it was for auth.quantum.ibm.com/api but the provider was still using the old subdomain, it has been updated in Updating URL subdomain #2081).
• the provider was not correctly specifying the Accept header to receive a JSON response and it was receiving an HTML response instead. This was updated in the provider in Add content/accept headers to appropriate http requests #2088.

It's fine to keep the retry logic, as long as at the end of the retries it displays the message in the response.

Acceptance criteria

• A Qiskit user using the ibm_quantum channel can see an exception message saying there is a planned maintenance ongoing.