Now checks if `unison` binary is installed before running

lib/vagrant/actions/vm/shared_folders.rb

@@ -57,10 +57,10 @@ module Vagrant
         def setup_unison
           return if unison_folders.empty?
 
-          logger.info "Setting up unison crontab..."
           runner.ssh.execute do |ssh|
             runner.system.prepare_unison(ssh)
 
+            logger.info "Creating unison crontab entries..."
             unison_folders.each do |name, data|
               runner.system.create_unison(ssh, data)
             end

lib/vagrant/systems/linux.rb

@@ -53,6 +53,8 @@ module Vagrant
       end
 
       def prepare_unison(ssh)
+        ssh.exec!("which unison", :error_key => :unison_not_found)
+
         logger.info "Preparing system for unison sync..."
         vm.ssh.upload!(StringIO.new(TemplateRenderer.render('/unison/script')), config.unison.script)
         ssh.exec!("sudo chmod +x #{config.unison.script}")

templates/strings.yml

@@ -212,6 +212,11 @@
 :system_unspecified: |-
   A VM system type must be specified! This is done via the `config.vm.system`
   configuration value. Please read the documentation online for more information.
+:unison_not_found: |-
+  The `unison` binary was not found on the guest machine. This is required
+  for folder syncing via unison. Please install unison on your system to
+  use folder syncing. You may also disable folder syncing, install unison,
+  re-enable syncing, then call a `vagrant reload` to enable syncing.
 :unknown_vm: |-
   The specified VM could not be found: <%= vm %>
 :virtualbox_import_failure: |-