test: include response body in non-JSON HTTP error msg

Useful for debugging issues.

Co-authored-by: Matias Furszyfer <matiasfurszyfer@protonmail.com>

test/functional/test_framework/authproxy.py

@@ -191,7 +191,7 @@ class AuthServiceProxy():
         content_type = http_response.getheader('Content-Type')
         if content_type != 'application/json':
             raise JSONRPCException(
-                {'code': -342, 'message': 'non-JSON HTTP response with \'%i %s\' from server' % (http_response.status, http_response.reason)},
+                {'code': -342, 'message': f"non-JSON HTTP response with \'{http_response.status} {http_response.reason}\' from server: {http_response.read().decode()}"},
                 http_response.status)
 
         data = http_response.read()