The refactor in 6e8f5d09 means that the
exception no longer exposes the name of the package with multiple versions,
and as a result the rescuer is unable to print this information.

Because we now have a path in which MultipleVersionsInstalledError doesn't
have the name at all, we can't reasonably restore the old behaviour.
And since rack resolution happens purely internal to the function that
raises the exception, the caller has no way to know what name to use.
However, since the exception text gets printed anyway, we can just move
this text into the exception itself.

Fixes the following error:

```
Error: mpd has multiple installed versions
Error: undefined method `name' for #<MultipleVersionsInstalledError:0x00007fc6009d8870>
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:137:in `rescue in uninstall'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:135:in `uninstall'
/usr/local/Homebrew/Library/Homebrew/brew.rb:119:in `<main>'
```