theme: improve legibility of error messages

- the login dialog uses warning_color, so be consistent
- nevertheless increase lightness of the error_color

Fixes issue https://gitlab.gnome.org/GNOME/gnome-shell/issues/1016

data/theme/gnome-shell-sass/_colors.scss

@@ -16,7 +16,7 @@ $link_visited_color: if($variant == 'light', darken($selected_bg_color, 20%), li
 $top_hilight: $borders_edge;
 
 $warning_color: #f57900;
-$error_color: #cc0000;
+$error_color: #ff8080;
 $success_color: if($variant == 'light', #33d17a, darken(#33d17a, 10%));
 $destructive_color: if($variant == 'light', #e01b24, darken(#e01b24, 10%));
 

data/theme/gnome-shell-sass/_common.scss

@@ -392,7 +392,7 @@ StScrollBar {
 
   .prompt-dialog-error-label {
     font-size: 10pt;
-    color: $error_color;
+    color: $warning_color;
     padding-bottom: 8px;
   }