When calculated a builtmethod, if it returns an expression with errors, then...

When calculated a builtmethod, if it returns an expression with errors, then the user doesn't have to print anything. Since we have an error state in Analyzer, then (to have uniform behaviour) we will print an empty string instead of 0. This way, if we have an error and the user prints the calculated result (by a bad use of API) then we will show an empty string instead of 0 (0 can't be understood as a computation, basically we tend to avoid misunderstandings in the use of API)