+1 for good long names

I would not combine the validations into one. Having them in separate methods gives each validation a name (method name). Also when good names are chosen a developer doesn't need to read body of the methods
```ruby 
before_destroy :confirm_not_library, :confirm_is_last_env
```
is usually enough.
 
If combined it would require comments and comments are just bad ;) They are never up to date and they are not first-class citizens in Ruby runtime. And it would violate single-responsibility principle.