more robust bootstrap: Isabelle-jEdit.shasum could be absent;

--- a/src/Pure/Tools/scala_project.scala	Sat Jan 02 23:03:47 2021 +0100
+++ b/src/Pure/Tools/scala_project.scala	Sun Jan 03 12:11:56 2021 +0100
@@ -38,6 +38,7 @@
           List(
             Path.explode("~~/lib/classes/Pure.shasum"),
             Path.explode("~~/src/Tools/jEdit/dist/Isabelle-jEdit.shasum"))
+        if path.is_file
         line <- Library.trim_split_lines(File.read(path))
         name =
           if (line.length > 42 && line(41) == '*') line.substring(42)