Fix unclear error message when deleting container

I was getting the following error:

```
$ bundles/0.7.6-dev/binary/docker-0.7.6-dev run -rm -v=/var/run:/foo base echo hi
hi
2014/01/28 14:24:46 Error: container_delete: No such id: run
```

This commit makes the true origin of the error clearer. Issue #3806 is
tracking the cause of the error.

Docker-DCO-1.1-Signed-off-by: Peter Waller <p@pwaller.net> (github: pwaller)
exexdriver: Make Command.GetExitCode an internal call

This code only works for backends that directly spawn the child
via the Command. It will not work for the libvirt backend. So
we move this code into the individual backends that need it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3805 from alexlarsson/execdriver-drop-getexitcode

exexdriver: Make Command.GetExitCode an internal call
Merge pull request #3807 from pwaller/fix-unclear-error

Fix unclear error message when deleting container