Allow `K"error"` when parsing to SyntaxTree (#60327)
I had assumed a valid parse since parse errors would detonate in lowering
anyway, but that was wrong: JETLS uses `ignore_errors=true` in parsing to
SyntaxTree. I plan to make some larger changes where I'll make sure
`RawGreenNode->SyntaxTree` is more thoroughly tested, but this is a
quick fix to get JETLS working.
