Improve superstruct error messaging in `snaps-rpc-methods`

This mostly involves work around deeply nested struct definitions i.e. unions of unions. We need a way to reliably throw the correct error and perhaps give a diff of thrown errors all at once.

Start in the snaps-rpc-methods code handling deeply nested struct definitions, especially unions of unions, and trace how validation errors are currently thrown. Define the expected behavior for reliably selecting the correct error and reporting a diff of all thrown errors; the work is done when those cases produce consistent, comprehensive messaging.

Written by the indexing model from the issue text.