resolves lessonly/scim_rails#23

Why?
The scim engine returns a custom response to the caller in the event
that there is a 500 error in the engine. It rescues any standard error
in order to do this.

Unfortunately this means that when something is broken it does not
bubble up to the parent app's error handling system or even be printed
in the logs.

We cannot just re-raise the exception because you cannot return a
response AND raise an exception as a part of the same request.

To help, this PR will make is possible for you to supply a callable
object that take the exception as it's argument.

If no callable object is provided, we will output the exception to the
logs so that silence is not the default.

To get the old behavior of completely ignoring an exception, you could
supply an empty proc.