[6.12.z] Add json response to entity creation error message

[Satellite-QE]

Cherrypick of PR: #1119

Description of changes

Add json response to entity creation error message

• Makes the error message information more detailed and informative
• Possibility to write more precise assertions

There are some robottelo tests that would greatly benefit from more precise assertions on error messages, i.e.
tests/foreman/api/test_repository.py::TestRepository::test_negative_update_checksum_with_on_demand_policy (or other tests in this module)
I doubt that the supposed error message should be:

RuntimeError: The repository's publication is missing. Please run a 'complete sync' on MPvytWJ.","errors":["Task c7fa616b-2f71-4476-b581-1cae65a9eafd: RuntimeError: The repository's publication is missing. Please run a 'complete sync' on MPvytWJ."]}

Upstream API documentation, plugin, or feature links

None

Functional demonstration

Example:

before:

requests.exceptions.HTTPError: ('422 Client Error: Unprocessable Content for url: https://robottelo...')

after

requests.exceptions.HTTPError: ('422 Client Error: Unprocessable Content for url: https://...', {'displayMessage': 'Validation failed: When "Upstream URL" is set, "Releases/Distributions" must also be set!', 'errors': {'base': ['When "Upstream URL" is set, "Releases/Distributions" must also be set!']}}

Additional Information

None

[Satellite-QE]

trigger: test-robottelo
pytest: -k test_negative_update_checksum_with_on_demand_policy tests/foreman/api/test_repository.py

[Satellite-QE]

PRT Result

Build Number: 27
Build Status: UNSTABLE
PRT Comment: pytest -k test_negative_update_checksum_with_on_demand_policy tests/foreman/api/test_repository.py --external-logging
Test Result : ==== 1 failed, 1 passed, 167 deselected, 205 warnings in 799.79s (0:13:19) =====