Force generators to abort if they fail.

Rails does not seem to provide a clean way to abort something if a
generator fails. Under most circumstances this is probably the proper
solution.

However, for us, these generators calls also function as smoke tests for
our actual generator code. If one of them fails, we need to fail the
build. Thankfully, if the generator had a problem the status is captured
in `$?`. We can verify that the last process was successful and abort
when it is not; thus failing the build and notifying the team.

example_app_generator/generate_stuff.rb

@@ -49,6 +49,11 @@ def self.environment_hooks
   end
 end
 
+def generate(*)
+  super
+  $?.success? || abort
+end
+
 # Generally polluting `main` is bad as it monkey patches all objects. In this
 # context, `self` is an _instance_ of a `Rails::Generators::AppGenerator`. So
 # this won't pollute anything.