Handle the case a plugin install is a plugin not found

lib/vagrant/errors.rb

@@ -344,6 +344,10 @@ module Vagrant
       error_key(:plugin_install_bad_entry_point)
     end
 
+    class PluginInstallNotFound < VagrantError
+      error_key(:plugin_install_not_found)
+    end
+
     class PluginLoadError < VagrantError
       status_code(81)
       error_key(:plugin_load_error)

plugins/commands/plugin/action/install_gem.rb

@@ -22,7 +22,13 @@ module VagrantPlugins
                                :name => plugin_name))
           installed_gems = env[:gem_helper].with_environment do
             installer = Gem::DependencyInstaller.new(:document => [])
-            installer.install(plugin_name)
+
+            begin
+              installer.install(plugin_name)
+            rescue Gem::GemNotFoundException
+              raise Vagrant::Errors::PluginInstallNotFound,
+                :name => plugin_name
+            end
           end
 
           # The plugin spec is the last installed gem since RubyGems

templates/locales/en.yml

@@ -175,6 +175,9 @@ en:
         the entry point doesn't exist. The entry point attempted was
         '%{entry_point}'. If this is not correct, please manually
         specify an `--entry-point` when installing the plugin.
+      plugin_install_not_found: |-
+        The plugin '%{name}' could not be found in local or remote
+        repositories. Please check the name of the plugin and try again.
       plugin_load_error: |-
         The plugin "%{plugin}" could not be found. Please make sure that it is
         properly installed via `vagrant plugin`.