core: warn if no provider is specified that we can't check for it

lib/vagrant/action/builtin/box_add.rb

@@ -33,7 +33,10 @@ module Vagrant
 
           # Determine if we already have the box before downloading
           # it again. We can only do this if we specify a format
-          if box_formats
+          if !box_formats
+              env[:ui].warn(I18n.t(
+                "vagrant.actions.box.download.no_provider_cant_check"))
+          else
             begin
               if env[:box_collection].find(box_name, box_formats)
                 raise Errors::BoxAlreadyExists,

templates/locales/en.yml

@@ -1213,6 +1213,11 @@ en:
           download_failed: |-
             Download failed. Will try another box URL if there is one.
           interrupted: "Box download was interrupted. Exiting."
+          no_provider_cant_check: |-
+            A provider wasn't explicitly provided to the "box add" command.
+            Without a provider, Vagrant can't verify if this box already
+            exists until the box is fully downloaded. This is just a warning,
+            and is not an error.
           resuming: "Box download is resuming from prior download progress"
         verify:
           verifying: "Verifying box..."