HPCC-12529 Report an error for an unknown %symbol%

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6630 from ghalliday/issue12529

HPCC-12529 Report an error for an unknown %symbol%

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>