Bug 21990: Replace dialog .error with .alert

Bug 21990: Replace dialog .error with .alert

<div class="dialog error"> will display a div without background color, the correct class is "dialog alert".

https://wiki.koha-community.org/wiki/Interface_patterns#Error_messages

Test plan:
Hit /cgi-bin/koha/acqui/parcel.pl
Notice the background color change with and without this patch

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>