install:addon command now throws a friendly error when it runs.

lib/commands/install-addon.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var Command     = require('../models/command');
+var SilentError = require('../errors/silent');
+var Promise     = require('../ext/promise');
+
+module.exports = Command.extend({
+  name: 'install:addon',
+  description: 'Deprecated. You can use `ember install` for ember-cli addons.',
+  works: 'insideProject',
+
+  anonymousOptions: [
+    '<addon-name>'
+  ],
+
+  run: function() {
+    var err  = 'This command has been deprecated. Please use `ember install ';
+    err     += '<addonName>` instead.';
+    return Promise.reject(new SilentError(err));
+  }
+});

lib/commands/install-npm.js

@@ -6,7 +6,7 @@ var Promise     = require('../ext/promise');
 
 module.exports = Command.extend({
   name: 'install:npm',
-  description: 'Npm package installs are now managed by the user.',
+  description: 'Deprecated. Npm package installs are now managed by the user.',
   works: 'insideProject',
 
   anonymousOptions: [

tests/unit/commands/install-addon-test.js

@@ -0,0 +1,47 @@
+'use strict';
+
+var expect = require('chai').expect;
+var InstallAddonCommand = require('../../../lib/commands/install-addon');
+var commandOptions = require('../../factories/command-options');
+
+describe('install:addon command', function() {
+  var command, options, msg;
+  before(function() {
+    options = commandOptions({
+      settings: {},
+      project: {
+        name: function() {
+          return 'some-random-name';
+        },
+        isEmberCLIProject: function() {
+          return true;
+        }
+      }
+    });
+
+    command = new InstallAddonCommand(options);
+
+    msg  = 'This command has been deprecated. Please use `ember install ';
+    msg += '<addonName>` instead.';
+  });
+
+  describe('with args', function() {
+    it('gives a helpful message if no arguments are passed', function() {
+      return command.validateAndRun(['ember-cli-cordova']).then(function() {
+        expect(false, 'should reject with error');
+      }).catch(function(err) {
+        expect(err.message).to.equal(msg, 'expect error to have a helpful message');
+      });
+    });
+  });
+
+  describe('without args', function() {
+    it('gives a helpful message if no arguments are passed', function() {
+      return command.validateAndRun([]).then(function() {
+        expect(false, 'should reject with error');
+      }).catch(function(err) {
+        expect(err.message).to.equal(msg, 'expect error to have a helpful message');
+      });
+    });
+  });
+});