This brings Python Joy into congruence with Nim.

It's hacky.  I edited the generated file.  The more complicated
functions like popop will not generate the same errors as the Nim
versions.  This is only congruence in the sense that the current jtest
suite passes identically on both.

Ideally I should be generating both the Nim and Python code from the
Prolog compiler.

joy/library.py

@@ -222,7 +222,7 @@ def BinaryBuiltinWrapper(f):
         try:
             (a, (b, stack)) = stack
         except ValueError:
-            raise StackUnderflowError
+            raise StackUnderflowError('Not enough values on stack.')
         if (not isinstance(a, int)
             or not isinstance(b, int)
             or isinstance(a, bool)  # Because bools are ints in Python.
@@ -849,7 +849,7 @@ def i(stack, expression, dictionary):
     try:
         quote, stack = stack
     except ValueError:
-        raise StackUnderflowError
+        raise StackUnderflowError('Not enough values on stack.')
     return stack, concat(quote, expression), dictionary
 
 
@@ -1174,7 +1174,7 @@ def dip(stack, expression, dictionary):
     try:
         (quote, (x, stack)) = stack
     except ValueError:
-        raise StackUnderflowError
+        raise StackUnderflowError('Not enough values on stack.')
     expression = (x, expression)
     return stack, concat(quote, expression), dictionary
 
@@ -1378,13 +1378,13 @@ def loop(stack, expression, dictionary):
     try:
         quote, stack = stack
     except ValueError:
-        raise StackUnderflowError
+        raise StackUnderflowError('Not enough values on stack.')
     if not isinstance(quote, tuple):
         raise NotAListError('Loop body not a list.')
     try:
         (flag, stack) = stack
     except ValueError:
-        raise StackUnderflowError
+        raise StackUnderflowError('Not enough values on stack.')
     if flag:
         expression = concat(quote, (quote, (S_loop, expression)))
     return stack, expression, dictionary