[v8r0] fix: dirac_dms_find_lfns: check if requested path exists and return error if not

[marianne013]

BEGINRELEASENOTES

*DataManagement FIX: dirac_dms_find_lfns: Check if requested path exists and returns error if not. Prevents users hitting #7487

ENDRELEASENOTES

[DIRACGridBot]

Sweep summary

Sweep ran in https://github.com/DIRACGrid/DIRAC/actions/runs/10809467057

Successful:

• integration