improve handling of uninstalled protocol handler
In https://bugzilla.mozilla.org/show_bug.cgi?id=1408010 (for Firefox 58), Mozilla fixed things so that the protocol handler code would prompt if a protocol handler application was configured and then later removed from the user's system. However, it seems that our patch for #22618 (moved) is incompatible with Mozilla's fix; with Tor Browser in this scenario, opening the protocol handler silently fails and a JS exception is logged to the browser console: NS_ERROR_FILE_NOT_FOUND: ... external-app-blocker.js:129 (external-app-blocker.js is part of Torbutton).
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