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)
Merge pull request #3807 from pwaller/fix-unclear-error

Fix unclear error message when deleting container