Nicer error if ansible-playbook is not installed on the host

lib/vagrant/errors.rb

@@ -87,6 +87,10 @@ module Vagrant
       error_key(:active_machine_with_different_provider)
     end
 
+    class AnsiblePlaybookAppNotFound < VagrantError
+      error_key(:ansible_playbook_app_not_found)
+    end
+
     class BaseVMNotFound < VagrantError
       error_key(:base_vm_not_found)
     end

plugins/provisioners/ansible/provisioner.rb

@@ -30,11 +30,15 @@ module VagrantPlugins
           :notify => [:stdout, :stderr]
         }
 
+        begin
           Vagrant::Util::Subprocess.execute(*command) do |type, data|
             if type == :stdout || type == :stderr
               @machine.env.ui.info(data.chomp, :prefix => false)
             end
           end
+        rescue Vagrant::Util::Subprocess::LaunchError
+          raise Vagrant::Errors::AnsiblePlaybookAppNotFound
+        end
       end
     end
   end

templates/locales/en.yml

@@ -66,6 +66,14 @@ en:
         Machine name: %{name}
         Active provider: %{active_provider}
         Requested provider: %{requested_provider}
+      ansible_playbook_app_not_found: |-
+        The "ansible-playbook" program could not be found! Please verify
+        that "ansible-playbook" is available on the PATH of your host
+        system, and try again.
+
+        If you haven't installed Ansible yet, please install Ansible
+        on your system. Vagrant can't do this for you in a safe, automated
+        way. Please see ansible.cc for more info.
       base_vm_not_found: The base VM with the name '%{name}' was not found.
       batch_multi_error: |-
         An error occurred while executing multiple actions in parallel.