HPCC-8987 - MoveExternalFile error misleading

MoveExternalFile gives and error when the target exists,
due to a bug however, when the error traced the source file name.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4128 from jakesmith/hpcc-8987

HPCC-8987 - MoveExternalFile error misleading

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>