GETINFO exit-policy for tor client should return 551

In #25423 (closed) it was noticed that...

[...] the client-only configuration ALWAYS returns this:

>>> GETINFO exit-policy/full
551 router_get_my_routerinfo returned NULL
}}}

The control spec didn't specify behavior for this case. For GETINFO it states: {{{ The server sends a 551 or 552 error on failure.

And more generically describes `551` and `552` as:

 551 Internal error
           [Something went wrong inside Tor, so that the client's
            request couldn't be fulfilled.]

 552 Unrecognized entity
           [A configuration key, a stream ID, circuit ID, event,
            mentioned in the command did not actually exist.]

Both `551` and `552` are a bit odd for this behavior, but the spec doesn't have a code that indicates "You asked for everything ok, but I'm not operating in that mode right now" (i.e. N/A).

However, the spec //does// indicate the following for `GETINFO fingerprint`:

"fingerprint" -- the contents of the fingerprint file that Tor
  writes as a relay, or a 551 if we're not a relay currently.
  (Added in 0.1.2.3-alpha)

In discussion with dgoulet and atagar over IRC, we agreed it made the most sense to follow this precedent.

dgoulet reports that `GETINFO fingerprint` currently returns:

551 Not running in server mode

The spec notes "Unless specified to have specific contents, the human-readable messages in error replies should not be relied upon to match those in this document.", so this info is mentioned to facilitate consistency.