ENH: Allow slicelets and Slicer tests to handle log messages

Two main applications:

• Errors are logged to file, which makes troubleshooting much easier.
• Errors can be captured during tests and the test can return with failure in case of unexpected errors occurred (work in progress).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24141 3bd1e089-480b-0410-8dfb-8563597acbee