Add 404 to improve user experience (#387)

* Add 404 to improve user experience when they load an extension from the wrong spot

* newline at eof

* .

Author: seanmakesgames

.gitignore

@@ -7,3 +7,5 @@ npm-debug.log
 
 # Ignore webpack generated files
 dist/
+
+.DS_Store

404.html

@@ -0,0 +1,6 @@
+No content was found at this location.<br/>
+If you are trying a sandbox sample, you need to use the sandbox server.<br/>
+<br/>
+npm run start-sandbox<br/>
+<br/>
+See the readme for more information.