Improve metrics dashboard validation error messages

What does this MR do?

Current error messages can be confusing, this MR improves error messages formatting in the spirit of original gem implementation https://github.com/davishmcclurg/json_schemer/blob/master/lib/json_schemer/errors.rb This code is not used yet by any user facing interface

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Does this MR meet the acceptance criteria?

Conformity

Not required - no user facing changes added:

- [ ] Changelog entry

- [ ] Documentation ([if required] (https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-required))

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