If the YouTube API JS fails to load, the rest of the JS should still run

Fixes BLD-472

cms/templates/base.html

@@ -80,10 +80,15 @@
             // externally hosted files
             "tender": "//edxedge.tenderapp.com/tender_widget",
             "mathjax": "//edx-static.s3.amazonaws.com/mathjax-MathJax-727332c/MathJax.js?config=TeX-MML-AM_HTMLorMML-full&delayStartupUntil=configured",
-            // youtube URL does not end in ".js". We add "?noext" to the path so
-            // that require.js adds the ".js" to the query component of the URL,
-            // and leaves the path component intact.
-            "youtube": "//www.youtube.com/player_api?noext"
+            "youtube": [
+                // youtube URL does not end in ".js". We add "?noext" to the path so
+                // that require.js adds the ".js" to the query component of the URL,
+                // and leaves the path component intact.
+                "//www.youtube.com/player_api?noext",
+                // if youtube fails to load, fallback on a local file
+                // so that require doesn't fall over
+                "js/src/youtube_fallback"
+            ]
         },
         shim: {
             "gettext": {

common/static/js/src/youtube_fallback.js

+console.error("Can't load YouTube -- anything that relies on it will fail");