[Analyzer] Enrich type unification errors with info about expected type

[Martinsos]

TypeChecker currently does typechecking in two main phases: unification (figuring out the strictes sub-type that works for all of the types in smth like e.g. list) and then weakening (figuring out if certain type can be "weakened" / coerced into a second (expected) type.
If an error happens during unification, we currently don't know in our implementation what is the expected type so we don't report that. We could do know it, it just not implemented that way.
If we wanted, we could refactor this piece of code so that information actually becomes available, probably by making phases of unification and weakening more interwined.

NOTE: If sum types (#381) get implemented, this probably won't be a concern any more, since there will likely be no more unification errors.