Check for dependency job stage index before comparison

What does this MR do?

This bug fixes #59366 (closed) - that referencing an not-yet-defined build stage in a dependent job throws an error when trying to compare the indexes of the stages to ensure that the dependent job is executed before the one defining the dependency. If the stage isn't found at all, it hasn't been previously defined and we should report the helpful error.

The configuration in question in #59366 (closed) is not valid and this change will not make it valid, but will allow us to report a meaningful error message about the configuration instead of breaking and returning a generic 500 response.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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