i3-msg: check reply in quiet mode

i3-msg: check reply in quiet mode

i3-msg currently exits right after sending the IPC message if the quiet
flag is set. This means that if an error occurred when issuing a
command, e.g. "i3-msg -q foobar", it gets silently ignored.

What we really want is to just skip printing but still check the reply.

At the same time, explicitly print the reply when we need to, instead of
using an exit label.