Prior to this commit, if Vagrant received checksum options from Vagrant
Cloud that were simply empty strings, it would try to validate its
checksum with those options. This commit fixes that by ignoring empty
string values.
Prior to this commit, the file_checksum class used the `Vagrant::Errors`
class as if it were apart of the Vagrant module. However, since the
file_checksum class is an interface and not part of the Vagrant module,
it doesn't have access to that Error class like other Vagrant modules.
This commit fixes that by ensuring the `"vagrant/errors"` class is
loaded, and that the proper namespace is used.
