i3bar: Amend status line error 127 message

Exit 127 can be returned by the shell when the command is not found or
when the `status_command` process returns 127 because of a missing C
library dependency.
Add `input_type` enum to `Binding` typedef

An input type of B_KEYBOARD will indicated this binding was created with
"bindsym", "bindcode", or "bind" and should only run on key press
events.

An input type of B_MOUSE will indicate this binding was created with
"bindmouse" and should only run on button press events (not yet
implemented).

For more information see #558.