improve error handling for loading

improve error handling for loading
I think all of the mussa file loading code throws a mussa_load_error
when something doesn't work. (instead of returning an error string).
All the GUIs catch this error and display the error in their message box

One annoying thing was that loading a different analysis after you'd already
loaded one in the Qt version caused it to try to merge the two versions
and then die when it ran its analysis. I got around that by deleting the
old mussa pointer and replacing it with the newly loaded version.
Though I discovered I need to do the pointer swap _after_ I'd successfully
loaded a the new analysis.

Also the Qt version doesn't try to load anything when the user presses
the cancel button in the load dialog boxes.