Add catch during manual yaml front matter editing

What does this MR do?

It:

1. Prevents yaml exceptions in the console during mid-edit of the yaml settings
2. It falls back to the last valid yaml front matter (if applicable) if the rare scenario below occurs:
   • user enters Markdown mode
   • user enters invalid front matter (someKey vs. someKey: with a requred value)
   • user switches to WYSIWYG mode

Without handling this #2 rare scenario, the front matter could be lost (technically overwritten w/undefined)

Screenshots

Before:

v2 After (current solution):

v1 After (old solution):

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #250343 (closed)