[BUG] Better error reporting for recursive operators

[konnov]

The following specification produces exceptions:

--------------------------- MODULE Rec10 -------------------------------------
EXTENDS Integers

VARIABLES f

RECURSIVE Fact(_)

Fact(n) ==
  IF n <= 1
  THEN 1
  ELSE n * Fact(n - 1)

Init == f = Fact(4)

Next == f' = Fact(7)

Inv == f >= 1
========================================================================

While the messages in the exceptions are right, they should be reported as normal errors. Additionally, a pointer to the manual should be given.