Chrome extension: Detect when access is denied

... during the deployment process, and redirect
the user to a page explaining the problem,
showing how to grant the "Allow access to file URIs"
permission to the extension.

Fixes #1663.
Warn on various deployment problems

... instead of failing silently.

 * When trying to deploy over the file:/// protocol
 * When trying to deploy over a PDF file, withouth PDF.js

Fixes #1650.