Metrics dashboard schema validation egde cases

What does this MR do?

It fixes number of edge cases, which were overlooked in initial MRs !33202 (diffs)

1. When dashboard has correct key which content has wrong type eg: :dashboard key with String in stead of a Hash 500 error was raised, now it should present validation warring
2. When dashboard schema processing at Gitlab::Metrics::Dashboard::Finder#find return error, schema was not loaded, and that caused to report false positive warnings eg:

dashboard: empty panels
panel_groups:

reported both :dashboard and :panel_groups as being blank, right now, only for purpose of validation schema is being reloaded, and processing is skipped in order to work that issue around.

Screenshots

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

Closes #210289 (closed)