src/Pure/Tools/build.scala

         case None => info.options
         case Some(n) => info.options.string("ML_process_policy") = NUMA.policy(n)
       }
 
     private val graph_file = Isabelle_System.tmp_file("session_graph", "pdf")
-    try { isabelle.graphview.Graph_File.write(options, graph_file, deps(name).session_graph) }
-    catch { case ERROR(_) => /*error should be exposed in ML*/ }
+    isabelle.graphview.Graph_File.write(options, graph_file, deps(name).session_graph)
 
     private val future_result: Future[Process_Result] =
       Future.thread("build") {
         val parent = info.parent.getOrElse("")
         val base = deps(name)