HPCC-11506 - Alert to contain existing files

Add name(s) of files to alert user what files are the ones that currently exist.

Signed-off by: Miguel Vazquez <miguel.vazquez@lexisnexis.com>
With that amount of files it can be an issue in any notification system. We can check if more than 10 files or so and then change the message within the alert to state: "more than x amount of files already exist"