Fail softly on .zshenv not existing

[fgblomqvist]

Even if you're using zsh you might not have a .zshenv file. It should probably check on start-up if such a file exist (and perhaps if .zshrc also exists) and tell the user, then terminate. Alternatively, it can create the .zshenv file if it doesn't exist, though that might be better left to the user perhaps.