ENH: Add debug option to SlicerWizard.py

Add an undocumented --debug option to SlicerWizard.py. When used,
exceptions are re-raised when caught, rather than showing a fatal error
with just the exception message. This is useful for development to get
the stack trace in case of unexpected exceptions due to incorrect code.
BUG: Don't catch die() in SlicerWizard.py

Separately catch SystemExit and re-raise, so that calls to die() from
within a try-except don't print strange error messages.
ENH: Add wrapped output to SlicerWizard.py

Add a 'printw' helper function to print wrapped text. Use this in die()
and one other appropriate spot.