[Propose] Raise Warning rather than Error if old version is already installed

This is a proposition. 

Currently, `brew install` would raise an error like

```
Error: zsh-5.0.0 already installed
To install this version, first `brew unlink zsh'
```

when the current latest version is not installed, but the LinkedKegs directory (/usr/local/Library/LinkedKegs/ruby-build/) exists, i.e., an old version is installed. 

However, when the current latest version is installed, `brew link` raises a warning like

```
Warning: zsh-5.0.4 already installed
```

Therefore, I propose to raise warning for when old version is already installed, too. I also feel

```
Error: zsh-5.0.0 already installed
To install this version, first `brew unlink zsh'
```

is not a critical error, so warning level is suitable. 

This change is especially useful to write Brewfile like:

```
install zsh
install ruby
...
```

because Error stops the execution of `brew bundle` at the line although Warning does not. 

cf. [Request] Want brew tap raises Warning rather than Error if already tapped https://github.com/Homebrew/homebrew/issues/25511