Exposing QL Syntax Errors

Checkpoint: making progress on improved error display when a QL
expression is broken.
Improved error message for the ____ operator
Better display of missing-end-quote errors

This works by disallowing "]]" in Text literals, so we produce an error
when we hit the end of the expression.
We now record which property we're working on in the Context
Syntax errors now show correctly in QL props