When the configured directory for FTP push has too many files, it will generate an exception and fail due to a stack overflow. When this happens just rescue out the exception and re-raise a custom error to provide some context to the user on the actual problem.
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.