Restore behaviour of missing image mode "None"

Restore behaviour of missing image mode "None"

Server defaults to a predefined image so it never "fails" unless
requested. That image is hiding failures and Face/X-Face default
handling was never triggered, so to avoid detecting such image (which
is prone to future error):

- Force a 404 error from server if not found
- Detect also HTTP errors on curl call

Also:
- Handle less-than-minimum files like errors and made it misses too