[GH-654] Missing condition and translations

lib/vagrant/util/subprocess.rb

@@ -27,7 +27,7 @@ module Vagrant
       def initialize(*command)
         @options = command.last.is_a?(Hash) ? command.pop : {}
         @command = command
-        @command[0] = Which.which(@command[0])
+        @command[0] = Which.which(@command[0]) unless File.exists? @command[0]
         unless @command[0]
           raise Errors::CommandUnavailableWindows if Platform.windows?
           raise Errors::CommandUnavailable

templates/locales/en.yml

@@ -167,6 +167,8 @@ en:
         available below.
 
         %{help}
+      command_unavailable: "command binary could not be found. Is this application installed?"
+      command_unavailable_windows: "command executable not found in any directories in the %PATH% variable."
       config_invalid: |-
         There are errors in the configuration of this machine. Please fix
         the following errors and try again: