One place for the checks that public entry points repeat. Every message
names the argument it is about, says what was actually wrong, and where
there is a relevant help topic it points at it. That is the voice of
as_sim_draw(), which is the model these follow: a user who mistypes an
argument should not have to guess which of theta and x the complaint is
about, or read a coercion error raised three frames down.
Details
These are internal helpers, not exported. Call them at public boundaries;
internal code that already knows its shapes keeps using as_theta_matrix().