minor feature enhancement to Mark IV pipeline

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Tom Droege recently explained to me that the pipeline will halt
in the middle of its work when it encounters a very cloudy image,
or an image taken as the sky brightens at dawn.  He asked if there
was some way to keep the pipeline going, despite such bad images,
so that all the good images could be reduced to completion.

  I have found at least some of the conditions which can cause
the pipeline to halt, and modified the main driver program so that
it handles the errors and attempts to continue instead of halting.
If people want to try out the fix, please do.  There was only
one file which changed: "markiv_driver.tcl".  You can download a 
new version at

         http://spiff.rit.edu/tass/pipeline/markiv_driver.tcl

Or, if you're compulsive, you can grab a tar file for the entire
pipeline and re-install:

         http://spiff.rit.edu/tass/pipeline/pipeline.tar.gz

  Feedback is _most_ welcome.  I'm not _sure_ that this will deal
with the problems Tom and others were having, but it might.
If problems persist, I can try to fix them, too.

                                      Michael Richmond