more robust: avoid race condition wrt. cleanup of ML process, e.g. relevant for "$ISABELLE_TMP/rat.ML" in theory Codegen.Further;

--- a/src/Pure/System/isabelle_system.scala	Thu Sep 13 16:15:05 2018 +0200
+++ b/src/Pure/System/isabelle_system.scala	Thu Sep 13 16:30:07 2018 +0200
@@ -197,14 +197,16 @@
         new SimpleFileVisitor[JPath] {
           override def visitFile(file: JPath, attrs: BasicFileAttributes): FileVisitResult =
           {
-            Files.deleteIfExists(file)
+            try { Files.deleteIfExists(file) }
+            catch { case _: IOException => }
             FileVisitResult.CONTINUE
           }
 
           override def postVisitDirectory(dir: JPath, e: IOException): FileVisitResult =
           {
             if (e == null) {
-              Files.deleteIfExists(dir)
+              try { Files.deleteIfExists(dir) }
+              catch { case _: IOException => }
               FileVisitResult.CONTINUE
             }
             else throw e