HPCC-11173 Fix roxie JSON Exception formatting

And fix some exceptions that were coming back as XML inside a JSON {}.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #5931 from afishbeck/jsonExceptionFormatting

HPCC-11173 Fix roxie JSON Exception formatting

Reviewed-By: Gordon Smith <gordon.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>