Don't pretend tor has bootstrapped when tor@default.service failed to start

Without this commit, this could — and would! — happen:

1. tor@default.service fails to start enough times for systemd to give up
   (fixed in !584)

2. tails-wait-until-tor-has-bootstrapped.service correctly fails thanks
   to BindsTo=tor@default.service

3. tails-tor-has-bootstrapped-flag-file.service starts and incorrectly
   tells the rest of the system, most notably Tor Connection
   and the Onion status icon in the top bar, that Tor is ready,
   while we still have "DisableNetwork 1" in torrc.

refs #18532