HPCC-8482 Delay reporting errors in field mappings until actually used

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-8482 Remove unreachable code

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4034 from ghalliday/issue8482

HPCC-8482 Delay reporting errors in field mappings until actually used

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>