HPCC-11506 - Alert to contain existing files

Add name(s) of files to alert user what files are the ones that currently exist.
Add "..more.." if more than 10 files are duplicates.

Previous Pull Request with comments.
https://github.com/hpcc-systems/HPCC-Platform/pull/6339#issuecomment-54300595

Signed-off by: Miguel Vazquez <miguel.vazquez@lexisnexis.com>
It might be better to put this in _Widget beside the other (similar) method.