Handle CaskAlreadyInstalledError more graceful

I'm using a [Brewfile](https://github.com/pstadler/dotfiles/blob/master/Caskfile) to install casks. Now the problem is that `brew bundle` exits if there's an error (i.e. non-zero exit code). To continue the execution of the Brewfile when a `CaskAlreadyInstalledError` happens, the `cask install` command should exit with 0.

Before:
```bash
$ brew bundle
Warning: brew-cask-0.24.0 already installed
Error: Cask for qlcolorcode is already installed. Use `--force` to install anyways.
Error: Command failed: L8:brew cask install qlcolorcode
```
After:
```bash
brew bundle   
Warning: brew-cask-0.24.0 already installed
Warning: Cask for qlcolorcode is already installed. Use `--force` to install anyways.
Warning: Cask for qlstephen is already installed. Use `--force` to install anyways.
Warning: Cask for qlmarkdown is already installed. Use `--force` to install anyways.
Warning: Cask for quicklook-json is already installed. Use `--force` to install anyways.
Warning: Cask for quicklook-csv is already installed. Use `--force` to install anyways.
Warning: Cask for betterzipql is already installed. Use `--force` to install anyways.
```