Bug 26886: Correct style of alert-error class in the OPAC

Bug 26886: Correct style of alert-error class in the OPAC

This patch makes a minor change to the OPAC SCSS so that instances of
"alert-error" classes are styled the same as messages with the Bootstrap
4 "alert-warning" class. This change is simpler than making multiple
changes to scripts and templates.

To test, apply the patch and try to view a single news item in the OPAC
which doesn't exist:

/cgi-bin/koha/opac-main.pl?news_id=XXX

- You should see a warning message with a pale yellow background, "This
  news item does not exist."
- Submit a suggestion, and then submit another suggestion with the
  same title.
  - You should see a warning message, "The suggestion has not been
  added. A suggestion with this title already exists."

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>