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>

koha-tmpl/opac-tmpl/bootstrap/css/src/_common.scss

@@ -187,11 +187,16 @@ h1 {
     padding: 1rem 0;
 }
 
+
 .alert {
     &.alert-info,
     &.alert-warning {
         color: #000;
     }
+
+    &.alert-error {
+        @extend .alert-warning
+    }
 }
 
 .btn-primary {