filter only [Extension] console output for the ExtensionLoader tests

This fix will improve the reliability of this test suite, since we are only checking for extension loader error message in the console output.
If some other code calls `console.error()` with null or a non-string, this line would crash. I guess the test would fail either way, but probably better to have it cleanly report it below if possible.  Maybe just check `typeof arguments[0] === "string"`?