BUG: Explicitly tell the user if extensions can't be installed

In case either the Slicer settings file or, the extensions install
directory are not readable/writable, a nicely formatted message will
be displayed to the user instead of the app-store page,

Note that the associated messages are also logged.

Fixes #3179

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22487 3bd1e089-480b-0410-8dfb-8563597acbee