[Build] Fixed ErrorHelper local variable

We need to emit a ldloca instruction before constructing the
ErrorHelper instance, in order to reference it in the finally
block below.

Source/Generator.Rewrite/Program.cs

@@ -363,9 +363,9 @@ static DebugVariables EmitDebugPrologue(MethodDefinition wrapper, ILProcessor il
 
                         vars.ErrorHelperLocal = new VariableDefinition(vars.ErrorHelperType);
 
-                        // var helper = new ErrorHelper(GraphicsContext.CurrentContext);
+                        // using (new ErrorHelper(GraphicsContext.CurrentContext)) { ...
                         il.Body.Variables.Add(vars.ErrorHelperLocal);
-                        //il.Emit(OpCodes.Ldarga, vars.ErrorHelperLocal); // todo: fix this
+                        il.Emit(OpCodes.Ldloca, vars.ErrorHelperLocal);
                         il.Emit(OpCodes.Call, vars.Get_CurrentContext);
                         il.Emit(OpCodes.Call, ctor);