fix: clear error when wkg get output directory does not exist

[cargopete]

Fixes #118.

When wkg get -o some_dir/ is used and some_dir doesn't exist, the previous error was:

Error: No such file or directory (os error 2) at path "/path/to/some_dir/.wkg-getT7LRZK"

The temp file name in the path sends the user in the wrong direction. The fix checks that parent_dir exists before attempting to create the temp file, and reports a clear error:

Error: output directory "/path/to/some_dir/" does not exist; create it first or choose a different path

[lann]

Thanks!