Add error attribute for access to thrown exceptions

[troyk]

Currently exceptions are converted to http status codes and the exception is swallowed. I might be missing something, but I need access to the exception. Let me know if you'd rather see the API implemented differently.

btw, lotus looks stellar and the performance with ruby 2.1.1 is inline with nodejs

[jodosha]

@troyk Thanks for noticing this missing behavior, it's a good point.

However, I don't like the design. That #error method returns the last raised error. If another exception is raised, that reference would be lost.

I know that right now Controller's exception management short circuits the execution with throw so that, the control returns immediately to the framework and theoretically we shouldn't have new exceptions to trace. But I don't wont a public API method (#error) to rely on this internal mechanism. Also, I'm reluctant to expand the API if I'm not 100% sure. I hope this makes sense for you.

Rack protocol has a solution for this problem: rack.errors.