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.
diff --git a/src/version.c b/src/version.c
index 311892c..c9a4feb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1604,
+/**/
     1603,
 /**/
     1602,