container start wrong error check

[rdallman]

noticed that when checking for if the user has entered the escape sequence
after starting the container, the code was checking that the error from
inspect was a term.EscapeError rather than the error from attach in order to
mask the error returned to a nil one - I think this is the error that was
intended to be checked here.

I did not add any tests, sorry, and I'm just gonna go with the social experiment to see
if I can get away with it first ;)

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "fix-attach-err-check" git@github.com:rdallman/cli-1.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Codecov Report

Merging #1914 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1914   +/-   ##
=======================================
  Coverage   56.71%   56.71%           
=======================================
  Files         310      310           
  Lines       21792    21792           
=======================================
  Hits        12359    12359           
  Misses       8518     8518           
  Partials      915      915