This submit command option will allow to check that the image (file)
actually exists in the specified url path (either locally if the
file:// prefix is used, or remotely), and if it doesnt, it immediately
exits with an APPLICATION_ERROR code (2).

This option doesn't check if the image is valid in some other way,
it only checks that an actual file exists in such a path.

It uses the python requests module to check for the existence of the
file remotely.

Reviewed By: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Maniphest Tasks: T823
Differential Revision: https://phabricator.collabora.co.uk/D300



Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>