Resolve "Parallel plotting hides relevant plotting error messages"

This MR fixes an issue where stderr (or anything else) was not displayed when doing parallel plotting via PlotManager. Now we're explicitly capturing stderr and showing it when there was an exception.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #381 (closed)