Bug: Exit with error, if the app is already there

[tsparber]

Description of enhancement

When installing an App that is already there, HBC should (in my opinion) exit with an error or warning.

Justification

The current behavior with the output message of "success" is misleading, as the app (probably a newer version) has not been successfully installed as it could not be moved.

Example use case

$ brew cask install firefox
==> Downloading https://ftp.mozilla.org/pub/firefox/releases/48.0/mac/en-US/Firefox%2048.0.dmg
Already downloaded: /Users/tommy/Library/Caches/Homebrew/firefox-48.0.dmg
==> Verifying checksum for Cask firefox
==> It seems there is already an App at '/Applications/Firefox.app'; not moving.
🍺  firefox was successfully installed!

(well, actually it was not)

[vitorgalvao]

Agreed. So much so, I consider the current behaviour a bug.

[frm]

I've been tackling this but I have one question regarding the intended behavior: what should happen if brew cask is invoked with multiple apps and one of them already exists?

Exiting with an error would not install the remaining apps.

[reitermarkus]

Ideally it would loop through all of them, and if one of them fails it is rescued with an error message, but the loop continues. Of course, the exit code at the end should be non-zero in this case.

Well, this should basically be the behaviour for any sort of install failure, not just when an app already exists.

[vitorgalvao]

Closing in favour of Homebrew/brew#1590.