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.

Did that in
https://github.com/hypothesis/h/commit/8f32229e1d54acd8f820fa02b320e926f40f2b0d.

Now we redirect to a shipped HTML document, which has some explanation text
and a screen-shot of the required setting.