Make stacktraces more readable

[jpsamaroo]

Anyone who's run into an error somewhere within a Mux app knows that the stacktraces are pretty inscrutable, due to the rather heavy usage of anonymous functions. We should find a way to make stacktraces more readable without compromising performance (such as the overhead of including lots of try/catch blocks) and composability.

In parallel with generating nicer stacktraces, we should also consider a "debug mode" which would allow better introspection into "message flow" through an app, potentially at the detriment of performance if necessary. Making it easy to use any of the JuliaDebug packages to debug requests and responses would be very useful, IMO.