error - generate a run-time error
void error( string err );Calling error(err) raises a driver/runtime error and stops normal execution
of the current evaluation path.
The driver routes uncaught errors through master error_handler(). If that apply returns a non-empty string, that string is emitted as error output. Otherwise the driver falls back to default debug/trace output.
When catch() traps the error, catch() receives the caught payload
according to its normal contract.