Fix behaviour of rmi -f with unexpected errors

[dnephin]

Carry #418
Closes#418
Closes#567

[codecov-io]

Codecov Report

Merging #654 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #654 +/- ##
==========================================
+ Coverage 50.02% 50.03% +0.01% 
==========================================
Files 216 216 Lines 17676 17680 +4 ==========================================
+ Hits 8842 8846 +4 
Misses 8391 8391 Partials 443 443

[dnephin]

First commit from #418 LGTM

[thaJeztah]

LGTM, thanks for carrying!

[vdemeester]

LGTM 🐮

[asottile]

Noticed this PR while trying to debug the following oddity (missing trailing newline):

$ docker --version
Docker version 17.09.0-ce, build afdb6d4
$ docker rmi doesnt_exist -f
Error: No such image: doesnt_exist$ 

This patch (or something related to it) fixes that but changes the exit code:

root@91136c2cdac6:/go/src/github.com/docker/docker# docker rmi -f doesnt_exist; echo $?
Error response from daemon: No such image: doesnt_exist:latest
1

From this it seems intentional, but perhaps unfortunate:
this is now inconsistent behaviour from what I would expect with (coreutils) rm ... vs rm -f ... (the latter exiting 0 even if the thing doesn't exist).

[dnephin]

This PR should actually fix the problem you see in 17.09. It'll be in 17.12