Chrome extension: Detect when access is denied

... during the deployment process, and display an error message
that might help the user the fix the problem by granting
the "Allow access to file URIs" permission to the extension.

Fixes #1663.
I think we need to explain this in more detail, I personally have no idea what "but forgot to enable "allow access to file URLs" at the extension page?" means...

Can we bundle a file in the extension that explains how to enable this and load that here instead.