Error message when a non-pure function is used as a constraint could be clearer

[bzbarsky]

Testcase:

  fn foo() -> bool { true }
  fn bar() : foo() {}

I get this error message: "error: Non-predicate in constraint: foo"

It would be nice if it just said something like "foo is not a pure function" instead.

[catamorphism]

Indeed, keep reporting those not-very-good error messages. Fix is coming.