tuned message, in accordance to ML side;

--- a/src/Pure/System/isabelle_system.scala	Fri Apr 24 15:02:12 2015 +0200
+++ b/src/Pure/System/isabelle_system.scala	Fri Apr 24 16:12:20 2015 +0200
@@ -145,7 +145,8 @@
   def getenv_strict(name: String): String =
   {
     val value = getenv(name)
-    if (value != "") value else error("Undefined environment variable: " + name)
+    if (value != "") value
+    else error("Undefined Isabelle environment variable: " + quote(name))
   }
 
   def get_cygwin_root(): String = getenv_strict("CYGWIN_ROOT")