script: Add message to SyntaxError (#39056)

Adding an optional message to be attached to a SyntaxError. Unblocks
#39050.

The enum definition of Syntax is now `Syntax(Option<String>)`. Future
PRs should probably add more appropriate messages to some of the
`Syntax(None)`s.

Testing: Just a refactor
Fixes: Partially #39053

Signed-off-by: Ashwin Naren <arihant2math@gmail.com>

components/script/dom/html/htmlelement.rs

@@ -757,7 +757,7 @@ impl HTMLElement {
             .nth(1)
             .is_some_and(|ch| ch.is_ascii_lowercase())
         {
-            return Err(Error::Syntax);
+            return Err(Error::Syntax(None));
         }
         self.as_element()
             .set_custom_attribute(to_snake_case(name), value, can_gc)