tuned error message;

--- a/src/Pure/System/isabelle_process.scala	Sat Mar 28 12:06:37 2020 +0100
+++ b/src/Pure/System/isabelle_process.scala	Sat Mar 28 13:30:38 2020 +0100
@@ -48,7 +48,9 @@
       case Session.Ready =>
         startup_error.fulfill("")
       case Session.Terminated(result) if !result.ok && !startup_error.is_finished =>
-        startup_error.fulfill("Session start failed: return code " + result.rc)
+        val syslog = session.syslog_content()
+        val err = "Session startup failed" + (if (syslog.isEmpty) "" else ":\n" + syslog)
+        startup_error.fulfill(err)
       case _ =>
     }