Fix salt provisioner error namespace to be more consistent with other provisioners

plugins/provisioners/salt/errors.rb

@@ -4,7 +4,7 @@ module VagrantPlugins
   module Salt
     module Errors
       class SaltError < Vagrant::Errors::VagrantError
-        error_namespace("salt")
+        error_namespace("vagrant.provisioners.salt")
       end
     end
   end