issue callable type error from dynamic_module_resolution if module is not an atom (#2345)

Author: mthom

src/lib/debug.pl

@@ -53,5 +53,4 @@
 %
 %  Generalize away Goal.
 
-
 *(_).

src/machine/system_calls.rs

@@ -1693,7 +1693,10 @@ impl Machine {
                 }
             }
             _ => {
-                unreachable!()
+                let call_form = functor!(atom!(":"), [cell(module_name), cell(self.machine_st.registers[2])]);
+                let stub = functor_stub(atom!("call"), narity + 1);
+                let err = self.machine_st.type_error(ValidType::Callable, call_form);
+                return Err(self.machine_st.error_form(err, stub));
             }
         );