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.
Fix broken generator.

The generator was failing due to the missing require.
Configure Action Mailer `preview_path` in railtie

This mirrors how Rails sets up the default `preview_path`. Using a
`before_initialize` allows the environments to be loaded but shims in
before Rails sets up its `test/` default.
Remove old implementation of setting the path.
Note ActionMailer preview updates in Changelog.
Fix how Rails 4.1 handles showing previews.
Remove skipped spec.

This is being tested via the rake task which uses the generated example
app. It would be nice to have a lighter weight method of testing the
railties in the main RSpec suite, until then keeping this :skull: spec
around just clutters the code base.
Merge pull request #1236 from rspec/move-preview_path-default

Move preview path default