Fix printing MultipleVersionsInstalledError details

[mistydemeo]

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The refactor in 6e8f5d0 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>'

New behaviour:

Error: mpd has multiple installed versions
Run `brew uninstall --force mpd` to remove all versions.

cc @whoiswillma

[whoiswillma]

Looks good to me. Thanks for catching this!