fix(library/unifier): tolerate exceptions in the type_checker::infer method. This can happen since when we try projections we don't check whether they are type correct

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug in process_delta

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug in process_delta