friendly errors if a non-existent gem install is tried

lib/vagrant/errors.rb

@@ -428,6 +428,10 @@ module Vagrant
       error_key(:plugin_gem_error)
     end
 
+    class PluginGemNotFound < VagrantError
+      error_key(:plugin_gem_not_found)
+    end
+
     class PluginInstallBadEntryPoint < VagrantError
       error_key(:plugin_install_bad_entry_point)
     end

lib/vagrant/plugin/manager.rb

@@ -40,6 +40,8 @@ module Vagrant
         @global_file.add_plugin(result.name)
 
         result
+      rescue ::Bundler::GemNotFound
+        raise Errors::PluginGemNotFound, name: name
       end
 
       # Uninstalls the plugin with the given name.

templates/locales/en.yml

@@ -474,6 +474,9 @@ en:
         manage Vagrant plugins. The output of the errors are shown below:
 
         %{output}
+      plugin_gem_not_found: |-
+        The plugin '%{name}' could not be installed because it could not
+        be found. Please double check the name and try again.
       plugin_install_bad_entry_point: |-
         Attempting to load the plugin '%{name}' failed, because
         the entry point doesn't exist. The entry point attempted was