synced_folders/rsync-auto: don't crash if rsync command fails [GH-4991]

plugins/synced_folders/rsync/command/rsync_auto.rb

@@ -191,6 +191,10 @@ module VagrantPlugins
                 # halt is happening. Just notify the user but don't fail out.
                 opts[:machine].ui.error(I18n.t(
                   "vagrant.rsync_communicator_not_ready_callback"))
+              rescue Vagrant::Errors::RSyncError => e
+                # Error executing rsync, so show an error
+                opts[:machine].ui.error(I18n.t(
+                  "vagrant.rsync_auto_rsync_error", message: e.to_s))
               end
             end
           end

templates/locales/en.yml

@@ -204,6 +204,12 @@ en:
       have specified `rsync_auto` to be false.
     rsync_auto_path: |-
       Watching: %{path}
+    rsync_auto_rsync_error: |-
+      There was an error while executing rsync. The error is shown below.
+      This may not be critical since rsync sometimes fails, but if this message
+      repeats, then please fix the issue:
+
+      %{message}
     rsync_communicator_not_ready: |-
       The machine is reporting that it is not ready for rsync to
       communicate with it. Verify that this machine is properly running.