core: nicer error if shorthand can't be found

lib/vagrant/action/builtin/box_add.rb

@@ -26,12 +26,25 @@ module Vagrant
 
           # If we received a shorthand URL ("mitchellh/precise64"),
           # then expand it properly.
+          expanded = false
           uri = URI.parse(url)
           if !uri.scheme && !File.file?(url)
+            expanded = true
             url = "#{Vagrant.server_url}/#{url}"
           end
 
-          if metadata_url?(url, env)
+          is_metadata = false
+          begin
+            is_metadata = metadata_url?(url, env)
+          rescue Errors::DownloaderError => e
+            raise if !expanded
+            raise Errors::BoxAddShortNotFound,
+              error: e.extra_data[:message],
+              name: env[:box_url],
+              url: url
+          end
+
+          if is_metadata
             add_from_metadata(url, env)
           else
             add_direct(url, env)

lib/vagrant/errors.rb

@@ -132,6 +132,10 @@ module Vagrant
       error_key(:box_add_no_matching_version)
     end
 
+    class BoxAddShortNotFound < VagrantError
+      error_key(:box_add_short_not_found)
+    end
+
     class BoxAlreadyExists < VagrantError
       error_key(:box_add_exists)
     end

templates/locales/en.yml

@@ -247,6 +247,13 @@ en:
         Address: %{url}
         Constraints: %{constraints}
         Available versions: %{versions}
+      box_add_short_not_found: |-
+        The box '%{name}' could not be found or could not be accessed
+        in the remote catalog. Please double-check the name. The
+        expanded URL and error message are shown below.
+
+        URL: %{url}
+        Error: %{error}
       boot_bad_state: |-
         The guest machine entered an invalid state while waiting for it
         to boot. Valid states are '%{valid}'. The machine is in the

test/unit/vagrant/action/builtin/box_add_test.rb

@@ -234,6 +234,21 @@ describe Vagrant::Action::Builtin::BoxAdd do
       end
     end
 
+    it "raises an error if shorthand is invalid" do
+      tf = Tempfile.new("foo")
+      tf.close
+
+      with_web_server(Pathname.new(tf.path)) do |port|
+        env[:box_url] = "mitchellh/precise64.json"
+
+        box_collection.should_receive(:add).never
+        app.should_receive(:call).never
+
+        expect { subject.call(env) }.
+          to raise_error(Vagrant::Errors::BoxAddShortNotFound)
+      end
+    end
+
     it "adds the latest version of a box with only one provider" do
       box_path = iso_env.box2_file(:virtualbox)
       tf = Tempfile.new("vagrant").tap do |f|