I like just rendering them similar to `shiny-output-error` but with a different class name. How about something like:

```
--- a/inst/www/shared/shiny.css
+++ b/inst/www/shared/shiny.css
@@ -13,11 +13,16 @@ table.data td[align=right] {
 
 .shiny-output-error {
   color: red;
+  white-space: pre-wrap;
 }
 .shiny-output-error:before {
   content: 'Error: ';
   font-weight: bold;
 }
+.shiny-output-validation-error {
+  white-space: pre-wrap;
+  color: #777;
+}
 
 .jslider {
   /* Fix jslider running into the control above it */
```