`ch-builder2tar`: no error if image doesn't exist in `ch-image` storage

E.g.:

$ export CH_BUILDER=ch-image
$ ch-builder2tar bar /var/tmp
builder: ch-image
exporting
/var/tmp/reidpr/ch-image/img/bar
bin/ch-builder2tar: 138: cd: can't cd to /var/tmp/reidpr/ch-image/img/bar
0.00 B 0:00:00 [<=>                                                            ]
compressing
0.00 B 0:00:00 [<=>                                                            ]
-rw-r----- 1 reidpr reidpr 20 Jan 26 10:51 /var/tmp/bar.tar.gz

It should stop with an error when the builder doesn't know about image bar.