blockdev: clean up error handling in do_open_tray

Returns negative error codes and accompanying error messages in cases where
the device has no tray or the tray is locked and isn't forced open. This
extra information should result in better flexibility in functions that
call do_open_tray.

Suggested by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Colin Lord <clord@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
