Better error message on too many inherited fields.

test/limit/many_inherited_fields.wren

@@ -136,7 +136,7 @@ class Foo {
   }
 }
 
-class Bar is Foo { // expect runtime error: A class may not have more than 255 fields, including inherited ones.
+class Bar is Foo {
   new {
     super
     _field129 = 129
@@ -266,15 +266,16 @@ class Bar is Foo { // expect runtime error: A class may not have more than 255 f
     _field253 = 253
     _field254 = 254
     _field255 = 255
-    _field256 = 256
   }
 
   bar {
     IO.print(_field129)
-    IO.print(_field256)
+    IO.print(_field255)
   }
 }
 
 var bar = new Bar
-bar.foo
-bar.bar
+bar.foo // expect: 1
+// expect: 128
+bar.bar // expect: 129
+// expect: 255