device: Restart verification if error is "retry"
fprintd's API docs say that "retry" errors for verification "the verification is still ongoing" and that "[the] user should retry scanning their finger. Unfortunately, retry errors are fatal in libfprint. Make fprintd restart operations when "retry" is the error for either identification or verification purposes. We need to also make sure that a "*Stop" D-Bus call will return as normal if called while we're stopping a verification or identification in order to restart it. Closes: #22