[RaphaelFakhri]

What does this PR do?

Fixes #261.

When the channel authorization or user authentication endpoint responds with a status of 400 or above, HttpURLConnection.getInputStream() throws (FileNotFoundException for 404, IOException: Server returned HTTP response code: 403 ... for other codes). The response body the server sent is lost, and the status check that follows the read never runs for these codes. The resulting AuthorizationFailureException or AuthenticationFailureException only carries the URL and status, not the reason the server gave.

This change in BaseHttpAuthClient:

• Reads the status code first and reads the body from getErrorStream() for statuses of 400 and above.
• Uses the response body as the exception message, as it already does for other non-2xx statuses.
• Falls back to Server returned HTTP response code: <status> when the error response has no body.

Successful responses are handled as before. New tests in HttpChannelAuthorizerTest and HttpUserAuthenticatorTest start a local HttpServer that returns 403 with a JSON body and check that the exception message is that body. Both tests fail on master and pass with this change.

CHANGELOG

• [FIXED] When the auth endpoint returns a status of 400 or above, the AuthorizationFailureException or AuthenticationFailureException message now contains the response body instead of only the URL (FileNotFoundException thrown when server responds with 400 or above #261).