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