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/css.rs

@@ -105,7 +105,9 @@ impl CSSMethods<crate::DomTypeHolder> for CSS {
                 RegisterPropertyError::InvalidSyntax |
                 RegisterPropertyError::InvalidInitialValue |
                 RegisterPropertyError::NoInitialValue |
-                RegisterPropertyError::InitialValueNotComputationallyIndependent => Error::Syntax,
+                RegisterPropertyError::InitialValueNotComputationallyIndependent => {
+                    Error::Syntax(None)
+                },
                 RegisterPropertyError::AlreadyRegistered => Error::InvalidModification,
             })?;