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/windowproxy.rs

@@ -536,7 +536,7 @@ impl WindowProxy {
                 .unwrap();
             let url = match existing_document.url().join(&url) {
                 Ok(url) => url,
-                Err(_) => return Err(Error::Syntax),
+                Err(_) => return Err(Error::Syntax(None)),
             };
             let referrer = if noreferrer {
                 Referrer::NoReferrer