src/Pure/Tools/isabelle_system.scala

   def getenv(name: String) = {
     val value = System.getenv(name)
     if (value != null) value else ""
   }
 
-  class BadVariable(val name: String) extends Exception
+  class BadVariable(val name: String) extends Exception {
+    override def toString = "BadVariable: " + name
+  }
 
   def getenv_strict(name: String) = {
     val value = getenv(name)
     if (value != "") value else throw new BadVariable(name)
   }