Improve reporting of known API calls error scenarios

[airwoodix]

Summary

Improve the reporting of errors from the remote server (portal or direct-access resource) propagated through HTTP error status codes.

For example, submitting a circuit on a direct-access backend that has less ions loaded than the circuit requires physical qubits is now reported as: (modulo changes in the backend's implementation)

Traceback (most recent call last):
  ...
httpx.HTTPStatusError: Client error '413 Request Entity Too Large' for url 'http://localhost:3001/api/v1/circuit/'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/413

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  ...
qiskit_aqt_provider.api_client.errors.APIError: Not enough qubits: available qubits=0, requested=4

Details and comments

The new APIError type is purposely not linked to httpx.HTTPStatusError, such that it can be extended to errors reported by the remote servers with success status codes (dedicated error payloads). For the moment, the interface only assumes that the detail object has an acceptable string representation.

[fg-aqt]

I would split the test_http_response_raise_for_status in test_errors.py up for easier readability. The test confirming that an APIError is raised should be separate from the one that checks that nothing is raised if the Response has a 2XX code.

[airwoodix]

Refactored test_http_response_raise_for_status in 8e89097.