Description of issue

It would be nice if brew cask doctor could provide a return code != 0 in case it detects an error like the following:

==> Homebrew-Cask Staging Location
/usr/local/Caskroom (error: not writable by current user)

Currently, brew cask doctor seems to return 0 in any case, which makes it harder for scripts calling it to detect an error situation. In comparison, brew doctor works as expected, giving a return code != 0 in case of error.