Be more clever about finding hh_server and handling errors
It was suggested internally a while ago that `hh_client` should
first look for an `hh_server` sitting right next to it, so you don't
have to muck with your `$PATH`. That's not hard to do, let's do it. Also
remove the very outdated `HH_HOME` mechanism I don't think anyone uses
anymore. If there isn't an `hh_server` next to the client, still try searching
the `$PATH`, because well why not. Needing to have everything on the
`$PATH` has already confused one person externally who was building from
source :\
While I'm at it, handle the case where we can't find `hh_server` a
little more gracefully than just trying to start it up in a loop while
`/bin/sh` yells on the terminal.
Reviewed By: @pikatchu
Differential Revision:
D1254953