#13 Add GUI file IO error notices

[Tom Armstrong]

I don't have a monster disk.
I have messed up a couple times and run out of disk space while saving the edited file and then, without checking first, deleting the original. Since the original is gone I'm no longer out of disk space and all seems normal -- until later when I try to watch the edited file and find it incomplete.
File IO error notices do get written to STDERR but I seldom run GOPchop from the command line so they effectively disappear into the ether.
This patch adds GUI warning dialogs for some file IO errors.

[Tom Armstrong]

Add GUI file IO error notices