TextRange: handle an error case
WIP: Add support for PDF url and fingerprint detection

Now the PDF plugin is able to properly detect the
URL and fingerprint of the PDF document being viewed
by PDF.js.

However, this support is not leveraged by Annotator core,
for now. (It's up the the application consuming Annotator
to activate it.)

Also, the current implementation should be amended
as https://github.com/hypothesis/h/issues/1407 is fixed.

(The pending change will break compatibility of the URL scheme,
so annotataions linked to PDF fingerprints will be LOST.)

Don't deploy this.