Fix a couple of accidentally f-less f-strings in MessageDialog

[SaschaCowley]

Link to issue number:

None

Summary of the issue:

A couple of strings introduced in the message dialog API work were supposed to be f-strings, but the f prefix was missed, making them string literals.

Description of user facing changes

Description of development approach

Changed the erroneous strings to f-strings.

Testing strategy:

Created a MessageDialog in the python console and ensured the debug and exception messages were output correctly.

Known issues with pull request:

None

Code Review Checklist:

• Documentation:
  • Change log entry
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• API is compatible with existing add-ons.
• Security precautions taken.

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