Right now it says "552 internal error" because there's no way for
getinfo_helper_*() countries to specify an error message.  This
patch changes the getinfo_helper_*() interface, and makes most of the
getinfo helpers give useful error messages in response to failures.

This should prevent recurrences of bug 1699, where a missing GeoIPFile
line in the torrc made GETINFO ip-to-county/* fail in a "not obvious
how to fix" way.