8199437: Improve diagnostic system assertion message in com.sun.net.httpserver impl

Reviewed-by: alanb, dfuchs

src/jdk.httpserver/share/classes/sun/net/httpserver/ServerImpl.java

@@ -407,7 +407,7 @@ class ServerImpl implements TimeSource {
                                     }
                                     handle (chan, conn);
                                 } else {
-                                    assert false;
+                                    assert false : "Unexpected non-readable key:" + key;
                                 }
                             } catch (CancelledKeyException e) {
                                 handleException(key, null);