Add missing message: security.book.count.max.msg

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/resources/messages.properties

@@ -141,4 +141,7 @@ button.page.logout				   = Sign out
 
 button.page.apiref                 = How to: REST API link references
 
-page.symbols.currency              = \u20AC
+page.symbols.currency              = \u20AC
+
+security.book.count.max.msg        = Unable to add more books. Book count limit is
+