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.