Prior to this commit, the `vagrant validate` command would only validate the first machine in a vagrant file. This commit improves that by validating all known machines in the environment. If one is not found, it will properly throw an exception instead of a stacktrace.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.