Remove extra margin from warning messages in authenticate page

In the public share authentication page the form elements appear inside
a container that uses the "warning" CSS class. When the given password
is wrong a warning message is shown inside that container; this message
uses the "warning" CSS class too, so the top margin set for ".warning"
elements need to be removed in that case.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>