preserve multiline log messages

[pkienzle]

Description

Preserve line breaks in error messages displayed in the log explorer.

Cleans up uncaught exception handling so that it appears in one place.

Minor unrelated code cleanup, removing an unused variable.

Fixes #3219

How Has This Been Tested?

Introduced an uncaught exception after plotting in the fitting widget so that uncaught exceptions could easily be generated (not included in the PR).

Created a plugin model with a c error so that compiler output could be checked.

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

[krzywon]

Looks fine. Haven't tested.