Prettier error messages

[Kacarott]

Might already be done, should confirm with a variety of failure types:

• Variable not declared
• Parse error
• Trying to recurse, when not in LetRec (same as Variable not declared?)
• Trying to call value provided by tests (" xxx is not a function")

... and others?

[JohanWiltink]

First three are already done I think; fourth can't be done.