Don't error out if a bucket is configured for a non-capable guest

Currently if you explicitly enable a bucket and the guest does not support it, a `RuntimeError` is raised. I would like to globally have `auto_detect` turned off (as I use a caching proxy for apt), but enable yum and gem caches. But then the yum bucket gets angry on debianoid boxes. :/

IMHO a warning message would be enough in these cases. I can make a PR if you agree.

If the consensus is against it, at least the raised error should at least be a subclass of `Vagrant::Errors::VagrantError` (see "ERROR HANDLING" in the [docs](http://docs.vagrantup.com/v2/plugins/development-basics.html)). And I would like to have something like this instead:
```ruby
config.cache.enable :yum, auto_detect: true
```