Need catch-all handler for early exceptions

[csatt]

Exceptions that are detected once mainloop() is running are logged and generate a dialog asking the user to send the log file. But (otherwise uncaught) exceptions that occur before mainloop() is running are not logged and are impossible to debug when the app is run with the standard pyinstaller executables. This was the case when a user encountered the problem reported in Issue #66. Fortunately, he was willing to run a special debug version of the app that started from the console, which allowed the stack trace to be seen.

What is needed is a catch-all exception handler that logs the stack trace and generates a dialog for otherwise uncaught exceptions that occur before mainloop() is running.

[csatt]

Since the exception could be detected even before logging is set up (and is by definition before the GUI is running), the stack trace cannot be "logged" and a dialog cannot be displayed. Instead, the stack trace should be written to a temporary file and the file should be opened with the system viewer.