Remove blocking mode when no hints found.
Remove blocking mode when no hints found.

This fixes an error in 39adee9090fc5aadfd5dd681f91b80025084858a.

Specifically, if there are no hints to select from, or no
documentElement, then link-hints mode exits without initiating an actual
mode (i.e. without calling its super() constructor).

With 39adee9090fc5aadfd5dd681f91b80025084858a, that leaves a mode in
place which blocks all keyboard events, thereby rendering Vimium
entirely hung.

See this line:

  https://github.com/philc/vimium/commit/39adee9090fc5aadfd5dd681f91b80025084858a#diff-e9abcb9ebcdb5af8e9f33651364673a1R59.

Here:

   - we explicitly remove the keyboard-blocking mode
   - we add exitOnEscape (so that the situation is at least
     recoverable), and
   - we add an indicator (so that we can see what's going on).

It is proposed that the indicator is a temporary feature, while the
global link hints feature is in shake down.

decaffeinate-examples/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,smblott-github/vimium,willsALMANJ/vimium,philc/vimium,philc/vimium,philc/vimium,poacher2k/vimium,decaffeinate-examples/vimium,smblott-github/vimium,smblott-github/vimium,mrmr1993/vimium,mrmr1993/vimium,poacher2k/vimium,mrmr1993/vimium,willsALMANJ/vimium,gdh1995/vimium,gdh1995/vimium,poacher2k/vimium,gdh1995/vimium