Add an error for when a referenced script does not have an UdonSharpB…

…ehaviour defined

- This was causing issues further down the line after compiling

Assets/UdonSharp/Editor/UdonSharpClassVisitor.cs

@@ -38,6 +38,9 @@ public override void VisitCompilationUnit(CompilationUnitSyntax node)
             }
 
             classDefinition.methodDefinitions = methodVisitor.definedMethods;
+
+            if (classCount == 0)
+                throw new System.Exception($"No UdonSharpBehaviour class found in script file, you must define an UdonSharpBehaviour class in a script referenced by and UdonSharpProgramAsset");
         }
 
         public override void VisitClassDeclaration(ClassDeclarationSyntax node)