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>