Inform the user about missing libraries

src/index.html

@@ -116,5 +116,35 @@
     <!-- All other scripts are loaded through require. -->
     <script src="thirdparty/require.js" data-main="brackets"></script>
 
+    <!-- Verify that all dependencies are loaded. -->
+    <script>
+        window.setTimeout(function () {
+            var deps = { "Mustache": window.Mustache, "jQuery": window.$, "CodeMirror": window.CodeMirror, "RequireJS": window.require };
+            var key, allOK = true;
+            for (key in deps) {
+                if (!deps[key]) {
+                    allOK = false;
+                    break;
+                }
+            }
+            if (allOK) {
+                return;
+            }
+            document.write("<h1>Missing libraries</h1>");
+            document.write("<p>Oops! One or more required libraries could not be found.</p>");
+            document.write("<ul>");
+            for (key in deps) {
+                if (!deps[key]) {
+                    document.write("<li>" + key + "</li>");
+                }
+            }
+            document.write("</ul>");
+            document.write("<p>If you are running from a local copy of the Brackets source, please make sure submodules are updated by running:</p>");
+            document.write("<pre>git submodule update --init</pre>");
+            document.write("<p>If you're still having problems, please contact us via one of the channels mentioned in the <a target=\"blank\" href=\"../README.md\">README</a></p>.");
+            document.write("<p><a href=\"#\" onclick=\"window.location.reload()\">Reload Brackets</a></p>");
+        }, 1000);
+    </script>
+
 </body>
 </html>