ENH: Update LandmarkRegistration to show warning if plugin failed to be loaded

See pieper/LandmarkRegistration@13d7b32

Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Wed Apr 8 00:20:27 2015 -0400

Display a warning when a built-in registration plugin failed to be loaded.

For example, if Slicer is build without SimpleITK support, the following
is displayed:

8<----8<----8<----8<----8<----8<----8<----8<----
[...]
Number of registered modules: 139
Registration: Failed to import 'LocalSimpleITK' plugin: No module named SimpleITK
Number of instantiated modules: 139
[...]
8<----8<----8<----8<----8<----8<----8<----8<----

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24164 3bd1e089-480b-0410-8dfb-8563597acbee