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/dissimilaroriginwindow.rs

@@ -236,7 +236,7 @@ impl DissimilarOriginWindow {
             "/" => Some(source_origin.clone()),
             url => match ServoUrl::parse(url) {
                 Ok(url) => Some(url.origin().clone()),
-                Err(_) => return Err(Error::Syntax),
+                Err(_) => return Err(Error::Syntax(None)),
             },
         };
         let msg = ScriptToConstellationMessage::PostMessage {