lib/sign: better error handling of ed25519 initialization
lib/sign: better error handling of ed25519 initialization

Add more precise error handling for ed25519 initialization.
Check the initialization status at the beginning of every public
function provided by ed25519 engine.

Signed-off-by: Denis Pynkin <3fbf95ee68899518ee8799d35f88c8f7982aa936@collabora.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree