i3-msg: only print input + errorposition if they exist

i3-msg: only print input + errorposition if they exist

Before:
$ i3-msg floating disable, move window to position 100 px 100 px
ERROR: Your command: (null)
ERROR:               (null)
ERROR: Cannot change position of a window/container because it is not floating.
[{"success":true},{"success":false,"error":"Cannot change position of a window/container because it is not floating."}]

After:
$ i3-msg floating disable, move window to position 100 px 100 px
ERROR: Cannot change position of a window/container because it is not floating.
[{"success":true},{"success":false,"error":"Cannot change position of a window/container because it is not floating."}]