src/Pure/Thy/sessions.scala

   def is_session_dir(dir: Path): Boolean =
     (dir + ROOT).is_file || (dir + ROOTS).is_file
 
   def check_session_dir(dir: Path): Path =
     if (is_session_dir(dir)) File.pwd() + dir.expand
-    else error("Bad session root directory (missing ROOT or ROOTS): " + dir.expand.toString)
+    else {
+      error("Bad session root directory: " + dir.expand.toString +
+        "\n  (missing \"ROOT\" or \"ROOTS\")")
+    }
 
   def directories(dirs: List[Path], select_dirs: List[Path]): List[(Boolean, Path)] = {
     val default_dirs = Components.directories().filter(is_session_dir)
     for { (select, dir) <- (default_dirs ::: dirs).map((false, _)) ::: select_dirs.map((true, _)) }
     yield (select, dir.canonical)