fenced_code.py: Implement error logging for hiliter.

[lemmasamaha]

Instead of returning error 400 when pygment library fails to parse we log the error and keep going

Fixes #22287

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[zulipbot]

Hello @zulip/server-markdown members, this pull request was labeled with the "area: markdown" label, so you may want to check it out!

[alya]

@alexmv are you the right person to review this one?

[timabbott]

@lemmasamaha can you post sample logging output from when this is triggered?

It would also be great to add a test, as mentioned in the issue.