Language server fails to accept 'fail' option after semantic predicate.

[raptor494]

ANTLR allows you to modify the error message of a failed semantic predicate by following it with a fail option, like so:

rule: alt1
    | {predicate}?<fail='message'>
    | {predicate}?<fail={"action "+"message"}>
    ;

The current language server implementation regards that fail option as a syntax error and does not attempt to compile the parser, even though it would successfully compile.

[mike-lischke]

I didn't even know this is possible o_O