plugin.core: _RequireLinksPostingMetadataHook can check several metadata.

Extend the base class from checking 1 metadata value to checking N.

This is preparation for RT#10643, letting payables be documented with
invoice or contract.

This does unify error reporting, because now we always report all
type/invalid value errors *plus* a missing error if appropriate.
I think this consistency and thoroughness is appropriate, although
it did require some adjustments to the tests.