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>
Doesn't address @richardkchapman original comment re 1000 matched items...

See https://github.com/hpcc-systems/HPCC-Platform/blob/master/esp/src/eclwatch/_Widget.js#L238 for what I did in arrayToList.