DeleteDirRecursively: Don't report error for absent directory

Check if the return value of std::filesystem::remove_all is -1 rather
than 0; the former is the specified return value if there's an error
while 0 just means the directory already didn't exist (which is the end
result we want).

Previously error messages such as the following were possible:
E[COMMON]: DeleteDirRecursively: [path]/User/RedirectSession/ failed The
operation completed successfully.

Also adds a period in the error string to make it look nicer.

0 files changed, 0 insertions, 0 deletions