Use cardinality-agnostic wording in error format.

Closes #1746.

core/lib/src/form/error.rs

@@ -381,7 +381,7 @@ impl crate::http::ext::IntoOwned for Errors<'_> {
 
 impl fmt::Display for Errors<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{} errors:", self.len())?;
+        write!(f, "{} error(s):", self.len())?;
         for error in self.iter() {
             write!(f, "\n{}", error)?;
         }