improved error message for missing key

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@5870 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/collection/Map.scala

@@ -53,7 +53,7 @@ trait Map[A, +B] extends AnyRef with PartialFunction[A, B] with Iterable[Pair[A,
      *  @return the value associated with the given key.
      */
     def apply(key: A): B = get(key) match {
-        case None => default
+        case None => default(key)
         case Some(value) => value
     }
     
@@ -174,7 +174,7 @@ trait Map[A, +B] extends AnyRef with PartialFunction[A, B] with Iterable[Pair[A,
      *  The method implemented here yields an error, 
      *  but it might be overridden in subclasses.
      */
-    def default: B = 
-      error("key not found")      
+    def default(key: A): B =
+      error("key not found: " + key)
 }