src/Pure/System/process_result.scala

--- a/src/Pure/System/process_result.scala	Fri Sep 07 17:08:47 2018 +0200
+++ b/src/Pure/System/process_result.scala	Fri Sep 07 19:11:16 2018 +0200
@@ -23,6 +23,8 @@
   def ok: Boolean = rc == 0
   def interrupted: Boolean = rc == Exn.Interrupt.return_code
 
+  def error_rc: Process_Result = if (interrupted) this else copy(rc = rc max 1)
+
   def check_rc(pred: Int => Boolean): Process_Result =
     if (pred(rc)) this
     else if (interrupted) throw Exn.Interrupt()