Only register sys.excepthook if the app actually started

[zas]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

Without this patch, when running tests the crash message box may be displayed.
The issue appeared in d796b1e

• JIRA ticket (optional): PICARD-XXX

Solution

With those changes, it will be displayed only if the app was actually started.

Action

Additional actions required:

• Update Picard documentation (please include a reference to this PR)
• Other (please specify below)

[phw]

Good move, LGTM

Now we only need.to figure out why the crash dialog sometimes shows up twice. I had this happening, too.