[IMP] web: reduce warning spam, only show rpc error warning in debug

Trigger a *lot* of spam while running tests

It's not great that this is now disabled:

* some cases are mistakes in the test e.g. RPC not properly setup,
  they should really be fixed
* others are literally testing error responses, the logging should be
  suppressed instead

Ideal scenario would probably be to upgrade this to `error` and have
an API of some sort to suppress it on a per-test basis (not unlike
`@mute_logger` on the Python side).

addons/web/static/tests/legacy/helpers/mock_server.js

@@ -166,7 +166,15 @@ var MockServer = Class.extend({
             var message = result && result.message;
             var event = result && result.event;
             var errorString = JSON.stringify(message || false);
-            console.warn('%c[rpc] response (error) ' + route, 'color: orange; font-weight: bold;', JSON.parse(errorString));
+            if (debug) {
+                console.warn(
+                    '%c[rpc] response (error) %s%s, during test %s',
+                    'color: orange; font-weight: bold;',
+                    route,
+                    message != null && ` -> ${errorString}`,
+                    JSON.stringify(QUnit.config.current.testName)
+                );
+            }
             return Promise.reject({message: errorString, event: event || $.Event()});
         });