HPCC-9684 - Issue better error message when writing to foreign

Various changes/refactoring meant, it was already better
in fact, before this commit, it complained about not being
able to remove a foreign.

This change makes it explicit that you can't write to a foreign

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5037 from jakesmith/hpcc-9684

HPCC-9684 - Issue better error message when writing to foreign

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>