ENHANCED: Better error handling if Type is invalid.

ENHANCED: Better error handling if Type is invalid.

Strictly speaking, type is currently not a type, so a domain error is
appropriate if Type is not a valid type. However, there are also other
cases where new types have been introduced in the past, and this
seems a good candidate for a new type. Let us hence use a type error.

Example:

    ?- can_be(listi, [a,b|Ls]).
    %@ ERROR: Type error: `type' expected, found `listi' (an atom)