Implement mechanism for function to return error message

[brodieG]

This is in the context of something like:

vet(numeric() && all_bw(., 0, 1)

where it would be nice if there was a mechanism to integrate the all_bw message. Right now we just get:

unitizer> vet(all_bw(., 0, 2), 1:3 + .01)
[1] "`all_bw(1:3 + 0.01, 0, 2)` is not TRUE (is \"character\" instead of a \"logical\")"

Might defer this to 0.2. One of the challenges here is that we need some kind of string structure that is meaningful standalone and also in the context of a vetr error.

One possible mechanism is to attach as an attribute to a FALSE return value. A little reluctant to assume that any string is just an error message (e.g. what about all.equal?).