titan checkout of a bad commit should not stop repository

[eschrock]

Expected Behavior

If I do a checkout of a non-existent commit, it shouldn't stop my repository

Actual Behavior

Titan will stop the repository before giving an error message, and leave the repository stopped.

Steps to Reproduce the Problem

$ titan checkout -c nosuchcommit hello-world
Stopping container hello-world
Checkout nosuchcommit
no such commit 'nosuchcommit' in repository 'hello-world'
$ titan ls
REPOSITORY            STATUS
hello-world           exited

Version

0.3.2