This commit (partially) removes a big source of inefficiency (at least for

This commit (partially) removes a big source of inefficiency (at least for
the select function and the rewrite tactic): CicUnification.UnificationFailure
exceptions were enriched producing (in a very expensive way) a nice error
message. However, the expensive error message is useful only when it must be
shown to the user. Very often this is not the case. Thus I am now postponing
the production of the error message, changing the type of the exception.
NOTE: this kind of improvement can be applied everywhere in the code!
At least it should be applied to functions that can normally fail (e.g.
unification, refinement, convertibility, type checking, etc.)