core: better error if attempting to uninstall system plugin

lib/vagrant/errors.rb

@@ -488,6 +488,10 @@ module Vagrant
       error_key(:plugin_state_file_not_parsable)
     end
 
+    class PluginUninstallSystem < VagrantError
+      error_key(:plugin_uninstall_system)
+    end
+
     class RSyncError < VagrantError
       error_key(:rsync_error)
     end

lib/vagrant/plugin/manager.rb

@@ -95,6 +95,13 @@ module Vagrant
       #
       # @param [String] name
       def uninstall_plugin(name)
+        if @system_file
+          if !@user_file.has_plugin?(name) && @system_file.has_plugin?(name)
+            raise Errors::PluginUninstallSystem,
+              name: name
+          end
+        end
+
         @user_file.remove_plugin(name)
 
         # Clean the environment, removing any old plugins

templates/locales/en.yml

@@ -730,6 +730,14 @@ en:
 
         Please remove the file and reinstall the plugins.
         If this error recurs, please report a bug.
+      plugin_uninstall_system: |-
+        The plugin you're attempting to uninstall ('%{name}') is a
+        system plugin. This means that the plugin is part of the installation
+        of Vagrant. These plugins cannot be removed.
+
+        You can however, install a plugin with the same name to replace
+        these plugins. User-installed plugins take priority over
+        system-installed plugins.
       provider_not_found: |-
         The provider '%{provider}' could not be found, but was requested to
         back the machine '%{machine}'. Please use a provider that exists.

test/unit/vagrant/plugin/manager_test.rb

@@ -149,6 +149,39 @@ describe Vagrant::Plugin::Manager do
       expect { subject.uninstall_plugin("foo") }.
         to raise_error(Vagrant::Errors::BundlerError)
     end
+
+    context "with a system file" do
+      let(:systems_path) { temporary_file }
+
+      before do
+        systems_path.unlink
+
+        described_class.stub(system_plugins_file: systems_path)
+
+        sf = Vagrant::Plugin::StateFile.new(systems_path)
+        sf.add_plugin("foo", version: "0.2.0")
+        sf.add_plugin("bar")
+      end
+
+      it "uninstalls the user plugin if it exists" do
+        sf = Vagrant::Plugin::StateFile.new(path)
+        sf.add_plugin("bar")
+
+        # Test
+        bundler.should_receive(:clean).once.with(anything)
+
+        # Remove it
+        subject.uninstall_plugin("bar")
+
+        plugins = subject.installed_plugins
+        expect(plugins["foo"]["system"]).to be_true
+      end
+
+      it "raises an error if uninstalling a system gem" do
+        expect { subject.uninstall_plugin("bar") }.
+          to raise_error(Vagrant::Errors::PluginUninstallSystem)
+      end
+    end
   end
 
   describe "#update_plugins" do