prune - Don't raise an exception if install_path doesn't exist

[mjoerussell]

I noticed that if you run shards prune in a project with the following two conditions:

1. shards.lock exists
2. The install directory (lib/) does not exist

Then the command raises an exception. This might not be a super common scenario, but in the context of CI it can be frustrating for this command to throw and then interrupt the whole job rather than just not doing anything.