src/Pure/PIDE/editor.scala

   sealed case class Output(
     snapshot: Document.Snapshot = Document.Snapshot.init,
     results: Command.Results = Command.Results.empty,
     messages: List[XML.Elem] = Nil,
     defined: Boolean = true
-  )
+  ) {
+    def proper: Boolean = messages.nonEmpty && defined
+  }
 }
 
 abstract class Editor[Context] {
   /* PIDE session and document model */