GUI error when looking for undistort files in the wrong directory

This isn't really a big issue, but we ran into this yesterday and it sent me off on a goose chase trying to track down the source of the problem, only to realize that it was something so easily resolved - so I think just having an error message print out in case someone else is ever in the same position, to just let them know that they need to change the undistort settings, would be a good thing. A simple fix to avoid future frustration from users.

What happened was that Kevin was trying to select the source position for some standard pivr experiments with petri dishes, so he was trying to use the distance to source function in the analysis tab. Every time he tried to do that he got an error relating to the gui looking for undistort files in the wrong directory, and then crashing because it can't find them. We were initially perplexed by why it was doing this but eventually figured out that he just had to change the undistort settings.