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/eventsource.rs

@@ -570,7 +570,7 @@ impl EventSourceMethods<crate::DomTypeHolder> for EventSource {
         let url_record = match base_url.join(&url) {
             Ok(u) => u,
             // Step 4 If urlRecord is failure, then throw a "SyntaxError" DOMException.
-            Err(_) => return Err(Error::Syntax),
+            Err(_) => return Err(Error::Syntax(None)),
         };
         // Step 1 Let ev be a new EventSource object.
         let ev = EventSource::new(