patch 9.0.1604: errors from the codestyle test are a bit confusing

Problem:    Errors from the codestyle test are a bit confusing.
Solution:   Use assert_report() with a clearer message.  Avoid a warning for
            an existing swap file.

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1604,
 /**/
     1603,
 /**/