Unsuffixed literals like 1 and 1.1, and free type parameters sometimes
have to be printed in error messages, which ended up with <V0>, <VI0>
and <VF0>. This change puts the words "generic" and "integer"/"float"
into the message so it's not a completely black box.
When a type error has already occurred, don't call ty::subst,
which may ICE due to the mismatch in the number of type params
involved.
I'm deeming this too small to review.
Closes#3680
