embed: Add debug when a web page is blocked

Should help with fixing bugs in an overzealous AdBlock.

https://bugzilla.gnome.org/show_bug.cgi?id=726883
embed: Fix crash with some password forms

31bc1fe6 transformed a simple escape hatch of a comparison into an
assertion. But that assertion keeps getting triggered, so it's clearly
the wrong option for now.

https://bugzilla.gnome.org/show_bug.cgi?id=723909
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