script: Add message to IndexSizeError (#40463)

Adding an optional message to be attached to a IndexSizeError.

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

Testing: Just a refactor
Fixes: Partially #39053

Signed-off-by: WaterWhisperer <waterwhisperer24@qq.com>

components/script/dom/css/cssrulelist.rs

@@ -37,7 +37,7 @@ impl Convert<Error> for RulesMutateError {
     fn convert(self) -> Error {
         match self {
             RulesMutateError::Syntax => Error::Syntax(None),
-            RulesMutateError::IndexSize => Error::IndexSize,
+            RulesMutateError::IndexSize => Error::IndexSize(None),
             RulesMutateError::HierarchyRequest => Error::HierarchyRequest,
             RulesMutateError::InvalidState => Error::InvalidState(None),
         }