show error when reporter processes die prematurely; make sure reporters are built for 10.7

All of the reporters were broken on 10.7 - oops :-(  I set the wrong deployment target.

Also, if a reporter dies before xctool is finished, we'll now show a nice error message.  We used to show nothing - xctool would try to `write()` to the reporter's pipe, which triggered a `SIGPIPE`, which made us exit immediately.