View Source: don't crash if toolkit path wasn't found - SL #4605

Log an error instead.

Toolbar class was considering a value of None for the path, but the
ViewSource class wasn't.
View Source: display toolkit source again - SL #4605

In case of a GTK activity, it will display the source of
sugar-toolkit-gtk3 .  We can enhace this later, if we could check if
the activity is GTK2 or GTK3.  There is a FIXME note in the code for
this.

In case of a web activity, it will display sugar_web inside the
activity bundle.  The initial selected file will be env.js, which is
analog to the initial file env.py in sugar-toolkit-gtk3 .
View Source: initiate with index.html for web activities

A better option would be js/activity.js, but that implies bigger
changes in FileViewer class, and we are in freeze.