core: only output error on batch action if not forked

lib/vagrant/batch_action.rb

@@ -90,10 +90,14 @@ module Vagrant
             # Store the exception that will be processed later
             Thread.current[:error] = e
 
+            # We can only do the things below if we do not fork, otherwise
+            # it'll hang the process.
+            if Process.pid == start_pid
               # Let the user know that this process had an error early
               # so that they see it while other things are happening.
               machine.ui.error(I18n.t("vagrant.general.batch_notify_error"))
             end
+          end
 
           # If we forked during the process run, we need to do a hard
           # exit here. Ruby's fork only copies the running process (which