Make brew-cask commands resilient to problems with a single Cask

[phinze]

We see issues like #178 on the regular, and their impact could be greatly reduced with a little bit of defensive coding around cask loading.

Problems with one cask shouldn't bring down the entire brew-cask universe.

[phinze]

with all the refactoring of late, i think we're a lot better off than we were before on this.

also this is a sort of vaguely defined issue, so i'm going to go ahead and close and we can always reopen more targeted issues if we see places for improvements.