HPCC-11150 File upload should give a warning if file exists.

Allow user to to overwrite file if it already exists in the dropzone.
If overwrite flag is not checked it will display a message to check
overwrite flag.

Signed-off by: Miguel Vazquez miguel.vazquez@lexisnexis.com
Merge pull request #5877 from miguelvazq/LandingZoneOverwrite

HPCC-11150 File upload should give a warning if file exists.

Reviewed-By: Gordon Smith <gordon.smith@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>