Kill the zeusclient only if something went wrong

Previously, the runner would kill the client process at_exit - this
introduced a race condition where the client would sometimes get the
async TERM signal before it could read the exit status, and exit with
a failure status.

While the reasoning behind killing the client is sound (if the runner
dies without the client noticing, the client could hang around
forever), it should only happen if there's an exceptional condition.

rubygem/lib/zeus.rb

@@ -118,14 +118,17 @@ def command(identifier, sock)
 
       kill_command_if_client_quits!(pid, client_pid)
 
-      at_exit{ Process.kill(:TERM, client_pid) }
-
       Process.wait(pid)
       code = $?.exitstatus || 0
 
       local.write "#{code}\0"
 
       local.close
+    rescue Exception
+      # If anything at all went wrong, kill the client - if anything
+      # went wrong before the runner can clean up, it might hang
+      # around forever.
+      Process.kill(:TERM, client_pid)
     end
 
     def kill_command_if_client_quits!(command_pid, client_pid)