Make sure to escape the user controlled URL passed as part of the error
message into the error404 template in order to avoid XSS.
Reported-by: 40826d <40826d@posteo.de>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from commit cd8bea94e61fa72a0a2ba7bc54d7b2d7b7572519)