type checking and error handling for builtins improved.

object/error.go

@@ -4,15 +4,22 @@ type Error struct {
 	Message string
 }
 
+func (e *Error) Bool() bool {
+	return false
+}
+
 func (e *Error) Type() ObjectType {
 	return ERROR_OBJ
 }
+
 func (e *Error) Inspect() string {
 	return "Error: " + e.Message
 }
+
 func (e *Error) Clone() Object {
 	return &Error{Message: e.Message}
 }
+
 func (e *Error) String() string {
 	return e.Message
 }