Fix Invalid CSS on Error-Pages (#17206).

Patch by Daniel Felix

git-svn-id: http://svn.redmine.org/redmine@13190 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merged r13167 (#16798).

git-svn-id: http://svn.redmine.org/redmine@13191 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merged r13168 (#16747).

git-svn-id: http://svn.redmine.org/redmine@13192 e93f8b46-1217-0410-a6f0-8f06a7374b81
Merged r13055 (#16519).

git-svn-id: http://svn.redmine.org/redmine@13193 e93f8b46-1217-0410-a6f0-8f06a7374b81