But we don't do that with any of the other (many) errors detected by later passes of the compiler. Those report dozens of SyntaxErrors, with good descriptive messages. Users can search the web for those messages too.

Also, I doubt that many people will ever get a TargetScopeError. The examples are all esoteric -- yes, the compiler needs to reject them, but no, they are not things one is likely to try intentionally. (The exception may be the forbidden form you're adding, [... for ... in (i := ...)].)