#597 "Invalid Command" is the wrong error message

[Behzad Salimi]

Please fix redefinition of already defined commands from "Invalid Command" to a much more useful message "Command ... already defined" for example, you added the built-in function zeta, you silently changed the demo files to define user-defined Zeta. A great deal of diagnostic time would be saved if instead of "invalid command" the code would say "command ... already defiined"