Issue when `bin/rails` does not exist

[schneems]

When the bin directory does not exist this CNB will still try to boot the server with bin/rails server. See the notes in heroku/jruby-getting-started#27 (comment)

It should instead:

• If a Procfile with a web item exists, warn
• If not, error

The message should contain:

• Description of the problem
• Explain how to generate these files, by running $ bundle exec rake app:update:bin