Display warning if vagrant command for trigger is not found

plugins/kernel_v2/config/vm_trigger.rb

@@ -113,6 +113,17 @@ module VagrantPlugins
       def validate(machine)
         errors = _detected_errors
 
+        commands = []
+        # TODO: Should this be cached...?
+        Vagrant.plugin("2").manager.commands.each do |key,data|
+          commands.push(key)
+        end
+
+        if !commands.include?(@command)
+          machine.ui.warn(I18n.t("vagrant.config.triggers.bad_command_warning",
+                                cmd: @command))
+        end
+
         if !@run.nil?
           if !@run.is_a?(Hash)
             # Run must be a hash

templates/locales/en.yml

@@ -1678,6 +1678,8 @@ en:
           The key `paranoid` is deprecated. Please use `verify_host_key`. Supported
           values are exactly the same, only the name of the option has changed.
       triggers:
+        bad_command_warning: |-
+          The command '%{cmd}' was not found for this trigger.
         name_bad_type: |-
           Invalid type set for `name` on trigger for command '%{cmd}'. `name` should be a String.
         on_error_bad_type: |-