buildx/monitor/commands
Kohei Tokunaga 437fe55104
monitor: improve error messages
Print more understandable messages on error:

- When ps fails because the monitor doesn't attach to any session, print "no
  attaching session" instead of "unknown ref".
- Avoid disconnect silently fails when the monitor doesn't attach to any
  session. Print "no attaching session" error instead.
- Fix error message of "attach"'s arguments. ("server name must be passed" ->
  "ID of session or process must be passed")

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
..
attach.go monitor: improve error messages
disconnect.go monitor: improve error messages
exec.go monitor: Move commands to a separated package
kill.go monitor: Move commands to a separated package
list.go monitor: Move commands to a separated package
ps.go monitor: improve error messages
reload.go monitor: extend monitor interface from controller
rollback.go monitor: Move commands to a separated package