Fail More Gracefully if Migration Files Are Missing

[hjc]

Right now, if you try to do an arnold up and an already migrated file isn't in your file system (because it's, say, in another branch), the command completely fails. It would be nice if we could somehow catch this, warn the user and possibly still procede with the migration.

I'm not real sure how feasible this is, but I do know my devs would love it! While I do know the code a bit, I'm not sure why this Exception is thrown when trying to go up (it makes sense when trying to go down).