add 404-specific error message

we trap 404s already because we want to bail out early on 404s, so
instead of displaying the typical bug report, display a more useful
banner to herd users in the right direction.
Pretty verbose error message but I'm somewhat concerned that this might be too advanced for some folks. 

Maybe something like:

```
Omnitruck artifact does not exist for version $version on platform $platform.

If you are bootstrapping a node, try setting knife[:bootstrap_version] to the most recent stable release of Chef or the major version number of Chef (11 or 12) you would like to use to fix this problem. 

You can find the list of stable releases on https://www.chef.io/download-chef-client/ by selecting your operating system and platform. 

If you are not bootstrapping it probably means either $platform is not supported or we have an issue in our systems. In either case, please create an issue on https://github.com/opscode/chef/issues/new with following info:

<<Any specific information we want to collect here?>>
```