Don't handle identity server failure as fatal, and use the right message

Fixes https://github.com/vector-im/riot-web/issues/10002

res/css/structures/auth/_Login.scss

@@ -67,6 +67,10 @@ limitations under the License.
     font-weight: normal;
 }
 
+.mx_Login_error.mx_Login_serverError.mx_Login_serverErrorNonFatal {
+    color: $orange-warning-color;
+}
+
 .mx_Login_type_container {
     display: flex;
     align-items: center;

res/themes/light/css/_light.scss

@@ -35,8 +35,9 @@ $selection-fg-color: $primary-bg-color;
 
 $focus-brightness: 105%;
 
-// red warning colour
-$warning-color: $notice-primary-color;
+// warning colours
+$warning-color: $notice-primary-color; // red
+$orange-warning-color: #ff8d13; // used for true warnings
 // background colour for warnings
 $warning-bg-color: #DF2A8B;
 $info-bg-color: #2A9EDF;