require most commands to succeed; cleanup on install failure

- add `run!` method which raises if command does not succeed
- use `run!` when the command we are running must succeed for things to
  move forward. this should help produce clearer error messages in
  failure scenarios.
- move caveats earlier in the install process so reports can be made
  about potential failures
- remove the destination tree on cask install failure, so the cask will
  not be considered installed
Merge pull request #1247 from phinze/dev/commands-must-succeed

require most commands to succeed; cleanup on install failure