Harden Composition pipeline to be more robust and transparent

[wendellpiez]

User Story:

Currently the Metaschema composition pipeline fails, and not helpfully, in predictable error conditions such as bad inputs.

On well-formed but incorrect inputs (not METASCHEMA or not the right namespace) it should not fail to produce legible outputs, even if only error messages.

Goals:

Revise metaschema composition logic so that a result is always given, even if sometimes an <ERROR>.

Dependencies:

None known. A documentation generation pipeline is a good place to test this, since a subpipeline that delivers not-XML yields a mean error ('template xsl:initial-template does not exist').

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.}