Better error handling for setting userdata values

UpsilonTests/UserDataTests.cs

@@ -26,6 +26,8 @@ namespace UpsilonTests
             public string FieldString = "TestField";
             public string FieldStringSet;
             private string _privateTestField = "hidden";
+            public bool GetOnly { get; } = false;
+            public bool PrivateSet { get; private set; } = false;
 
             public bool TestMethodHasRun { get; private set; }
             public void TestMethod()
@@ -121,5 +123,37 @@ end
             Assert.Single(script.Diagnostics.Messages);
         }
 
+        [Fact]
+        public void CantSetToFieldsWithNoSetter()
+        {
+            var obj = new UserDataHelper();
+            const string input = @"
+function test(o)
+    o.GetOnly = true
+end
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            script.EvaluateFunction("test", new[] {obj});
+            Assert.Single(script.Diagnostics.Messages);
+            Assert.False(obj.GetOnly);
+        }
+
+        [Fact]
+        public void CantSetToFieldsWithPrivateSetter()
+        {
+            var obj = new UserDataHelper();
+            const string input = @"
+function test(o)
+    o.PrivateSet = true
+end
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            script.EvaluateFunction("test", new[] {obj});
+            Assert.Single(script.Diagnostics.Messages);
+            Assert.False(obj.PrivateSet);
+        }
+
     }
 }