Re-word Linux RDP error to include `xfreerdp`.

Changed the name of the error LinuxRDesktopNotFound to
LinuxRDPClientNotFound and re-worded error text in
templates/locales/en.yml to include `xfreerdp` when listing supported
RDP clients.

lib/vagrant/errors.rb

@@ -408,8 +408,8 @@ module Vagrant
       error_key(:linux_nfs_mount_failed)
     end
 
-    class LinuxRDesktopNotFound < VagrantError
-      error_key(:linux_rdesktop_not_found)
+    class LinuxRDPClientNotFound < VagrantError
+      error_key(:linux_rdp_client_not_found)
     end
 
     class LocalDataDirectoryNotAccessible < VagrantError

plugins/hosts/linux/cap/rdp.rb

@@ -13,7 +13,7 @@ module VagrantPlugins
             elsif Vagrant::Util::Which.which("rdesktop")
               "rdesktop"
             else
-              raise Vagrant::Errors::LinuxRDesktopNotFound
+              raise Vagrant::Errors::LinuxRDPClientNotFound
             end
 
           args = []

templates/locales/en.yml

@@ -818,10 +818,11 @@ en:
         that the NFS client software is properly installed, and consult any resources
         specific to the linux distro you're using for more information on how to
         do this.
-      linux_rdesktop_not_found: |-
-        The `rdesktop` application was not found. Vagrant requires this
-        in order to connect via RDP to the Vagrant environment. Please ensure
-        this application is installed and available on the path and try again.
+      linux_rdp_client_not_found: |-
+        An appropriate RDP client was not found. Vagrant requires either
+        `xfreerdp` or `rdesktop` in order to connect via RDP to the Vagrant
+        environment. Please ensure one of these applications is installed and
+        available on the path and try again.
       machine_action_locked: |-
         An action '%{action}' was attempted on the machine '%{name}',
         but another process is already executing an action on the machine.