Fixed that JS warning is not displayed when attachment maximum size is 0 (#13949).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11841 e93f8b46-1217-0410-a6f0-8f06a7374b81
Display error messages when attachment validation fails (#13949).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11842 e93f8b46-1217-0410-a6f0-8f06a7374b81