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
COMP: Update ITKv4 project

$ git shortlog a8429da..97a08e0

Matthew McCormick (3):
      BUG: Enable MRC IO in UseITK whenever the ITKReview module is enabled.
      Merge branch 'MRC-UseITK' into release
      COMP: Fix NIFTI CMake variable separation warning.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22521 3bd1e089-480b-0410-8dfb-8563597acbee