WIP: improve error message when mermaid exception is raised

What does this MR do?

Fix the flash exception showed when there is a syntax error that prevents mermaid diagram to show up.

Address https://gitlab.com/gitlab-org/gitlab-ce/issues/59556

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers