Improve error reporting when non-existent index referenced

[aj-stein-nist]

User Story:

Per conversation after review NPE analysis and rubber duck debugging explained in #279, it would be helpful for developers using metaschema-java libraries or the CLI to receive precise error messaging when a model references an index that does not exist, for example when <index-has-key/> has a @name attribute that should reference an existing schema by name that is in fact non-existent.

Goals:

More precise error messaging to clarify the use of <index/> and <index-has-key/> usage alongside upcoming clarifications int he spec.

Dependencies:

N/A

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}