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/cssrulelist.rs

@@ -35,7 +35,7 @@ unsafe_no_jsmanaged_fields!(RulesSource);
 impl Convert<Error> for RulesMutateError {
     fn convert(self) -> Error {
         match self {
-            RulesMutateError::Syntax => Error::Syntax,
+            RulesMutateError::Syntax => Error::Syntax(None),
             RulesMutateError::IndexSize => Error::IndexSize,
             RulesMutateError::HierarchyRequest => Error::HierarchyRequest,
             RulesMutateError::InvalidState => Error::InvalidState,