[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/tests/expression/worst_fib.fine

@@ -4,10 +4,16 @@ fun worst_fib(n: f64) -> f64 {
   } else if n == 1 {
     1
   } else {
-    worst_fib(n-2) + worst_fib(n-1)
+    worst_fib(n-2) + delegate_worst_fib(n-1)
   }
 }
 
+// NOTE: This nonsense exists to make sure mutual recursion works, in
+//       addition to direct recursion.
+fun delegate_worst_fib(n: f64) -> f64 {
+  worst_fib(n)
+}
+
 fun test() -> f64 {
   worst_fib(10)
 }