BUG: Report dependent extension not found as warning

For importing path variables (library paths, python paths, etc) from dependent extensions, find_package() has to be called.
However, many existing extensions don't generate (ExtensionName)Config.cmake file yet, therefore for now don't fail with an error if the dependent extension is not found, just report it as a warning.

All extensions that other extension may depend on should have this line at the end of the top-level CMakeLists.txt:

include(\${Slicer_EXTENSION_GENERATE_CONFIG})

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25991 3bd1e089-480b-0410-8dfb-8563597acbee