Exit with status of 1 on error

Fixes #393. ~~Will rebase against master once changes from #1526 arrive.~~

Before:
```
~ $ brew cask install baz && brew cask install quux
Error: No available cask for baz
Error: No available cask for quux
```

After:
```
~ $ brew cask install baz && brew cask install quux
Error: No available cask for baz
~ $
```