mw.loader.using() should not throw exceptions when asked to load a non-existent module.

mw.loader.using( 'foo' ).then(
  function () { console.log( 'Loaded!' ); },
  function () { console.log( 'Failed.' ); }
);

Expected: "Failed." is logged to the console.

Actual: Uncaught Error: Unknown dependency: foo

My use case is creating a soft dependency on TitleBlacklist by attempting to load the 'mediawiki.api.titleblacklist' module and assuming nothing is blacklisted if the load fails.

mw.loader.load() doesn't throw an exception, but it doesn't have a callback/promise, so it's useless for me.