Improve duplicate table errors

[lqd]

This might have saved me some time when looking for the CI failure in #331 where a stray space broke the markdown parser into thinking there were multiple tables there but it was unclear as to why.

Before

Error: loading goal from `src/2025h2/scalable-vectors.md`

Caused by:
    multiple goal tables found in section `src/2025h2/scalable-vectors.md:143:4: Ownership and team asks`

After

Error: loading goal from `src/2025h2/scalable-vectors.md`

Caused by:
    markdown parsing unexpectedly encountered multiple (2) goal tables in section `src/2025h2/scalable-vectors.md:143:4: Ownership and team asks`:
    1: "Task, Owner(s) or team(s), Notes"
    2: "Implementation, @davidtwco, Part II of Sized Hierarchy implementation"

_{ngl the Spanned<String>s are not IDE-friendly / intuitive to work with}