app-tools compilation should be more clear about when it has failed to compile something

[avescodes]

As brought up in #105, sometimes we write code that can't actually be represented in ClojureScript (character manipulation, for example.) In these cases, app-tools generally serves a blank page instead of ones applications, and it is quite difficult to track down the source of these errors without scanning through log output.

We should degrade more gracefully in these cases. Ideally we might display an error page a la the Rails stack trace page: