Tweak no ORM check on InstallGenerator.

* Expand the explanation of why it fail.
* Raise a subclass of `Thor::Error` so the Thor doesn't output the exception
  backtrace as it isn't useful for developers facing this error.

lib/generators/devise/install_generator.rb

@@ -3,6 +3,8 @@ require 'securerandom'
 
 module Devise
   module Generators
+    MissingORMError = Class.new(Thor::Error)
+
     class InstallGenerator < Rails::Generators::Base
       source_root File.expand_path("../../templates", __FILE__)
 
@@ -10,7 +12,19 @@ module Devise
       class_option :orm
 
       def copy_initializer
-        raise "An ORM must be set to install Devise" unless options[:orm]
+        unless options[:orm]
+          raise MissingORMError, <<-ERROR.strip_heredoc
+          An ORM must be set to install Devise in your application.
+
+          Be sure to have an ORM like Active Record or Mongoid loaded in your
+          app or configure your own at `config/application.rb`.
+
+            config.generators do |g|
+              g.orm :your_orm_gem
+            end
+          ERROR
+        end
+
         template "devise.rb", "config/initializers/devise.rb"
       end
 

test/generators/install_generator_test.rb

@@ -5,17 +5,20 @@ class InstallGeneratorTest < Rails::Generators::TestCase
   destination File.expand_path("../../tmp", __FILE__)
   setup :prepare_destination
 
-  test "Assert all files are properly created" do
-    run_generator(['--orm=active_record'])
+  test "assert all files are properly created" do
+    run_generator(["--orm=active_record"])
     assert_file "config/initializers/devise.rb", /devise\/orm\/active_record/
     assert_file "config/locales/devise.en.yml"
   end
 
-  test "Fail if no ORM is specified" do
-    error = assert_raises RuntimeError do
+  test "fails if no ORM is specified" do
+    stderr = capture(:stderr) do
       run_generator
     end
 
-    assert_match /An ORM must be set to install Devise/, error.message
+    assert_match %r{An ORM must be set to install Devise}, stderr
+
+    assert_no_file "config/initializers/devise.rb"
+    assert_no_file "config/locales/devise.en.yml"
   end
 end