prbuf: fix "wrong argument type" error

ffi.typeof() returns cdata, which in turn can't be concatenated with string
without explicit call of `tostring`. Moreover, passing data of native Lua
type would also generate wrong error. So let's simply raise general error
without mentioning passed type.

NO_DOC=<Internal fix>
NO_CHANGELOG=<Internal fix>

(cherry picked from commit f715e36a)