mbedit and mbnavedit don't properly handle input file format errors

[tomoreilly]

Describe the bug
mbedit and mbnavedit don't handle input file format errors in a useful way. E.g. when specifying an ascii file named "junkFIle" on the command line:

mbedit -I junkFile : dialog says "Unable to open input file. You may not have permission in this directory!"
(stderr message is more useful: "Illegal format identifier, initialization failed")

mbnavedit -I junkFile: main app window is blank, with no error dialog or stderr message

To Reproduce
Create a small ascii file, e.g. named 'junk'. Provide junk as an input file to mbedit and mbnavedit.

Expected behavior
An error dialog message should appear, with appropriate message (e.g. "unreadable data in file 'junk'")