print generators on failed generate

Let's say we just ran:

```
$ rails g migrate add_click_to_issue_assignment
```

We will get an error that looks like:

```
Could not find generator migrate.
```

This patch adds all existing migrations to the output to make it easier for a developer to find a valid migration.

```
Could not find generator "migrate". Please select a valid generator:
Rails:
  assets
  controller
  generator
  helper
  integration_test
  mailer
  migration
  model
  resource
  scaffold
  scaffold_controller
  task
```


It would be nice to do some spelling detection and suggest alternatives, but for now this should  help.

1 files changed, 4 insertions, 2 deletions

diff --git a/railties/test/generators_test.rb b/railties/test/generators_test.rb
index eac28badfe..8d6dbf80c2 100644
--- a/railties/test/generators_test.rb
+++ b/railties/test/generators_test.rb
@@ -21,8 +21,10 @@ class GeneratorsTest < Rails::Generators::TestCase
   end
 
   def test_invoke_when_generator_is_not_found
-    output = capture(:stdout){ Rails::Generators.invoke :unknown }
-    assert_equal "Could not find generator unknown.\n", output
+    name = :unknown
+    output = capture(:stdout){ Rails::Generators.invoke name }
+    assert_match "Could not find generator '#{name}'", output
+    assert_match "scaffold", output
   end
 
   def test_help_when_a_generator_with_required_arguments_is_invoked_without_arguments