WOQL instantiation errors should return the bound variable name

[GavinMendelGleason]

Currently we get a rather uninformative error message referring to an internal unbound prolog variable. We need to do a backwards lookup in the environment and report this instead. This could be a bit tricky as we need to catch in a context in which the environment is exposed to us. Potentially we need to do a re-throw wrapper around the entire prolog term compiled so that we can ensure this.