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