Fix return overrides to override error bodies

[buger]

The previous PR #2731 contained an error, and always forced return custom error: which brakes backward compatibility.
In this PR I added tests covering all return overrides functionality and normalized its behavior for JSVM and Python.

Additionally, it use reverse logic now, and plugin needs set ReturnOverrides.OverrideError (JS) or return_overrides.override_error (Python) to true in order to override the body of response.

Additionally, I renamed response_error field to response_body, because it makes way more sense.
The old field still can be set for backward compatibility.

Example python middleware:

from tyk.decorators import *
from gateway import TykGateway as tyk

@Hook
def MyRequestHook(request, response, session, metadata, spec):
	request.object.return_overrides.headers['X-Foo'] = 'Bar'
	request.object.return_overrides.response_code = 501
        request.object.return_overrides.response_body = "custom error"
        request.object.return_overrides.override_error = True

	return request, session

#2693

[buger]

/release to release-2.9

[tykbot]

Working on it! Note that it can take a few minutes.

[tykbot]

@buger Seems like there is conflict and it require manual merge.