Preset execution flows to handle common errors and exceptions

[gdaniel]

It would be nice to add a mechanism to handle internal exception with bot actions. For example: when the DialogFlow connector throws an exception INVALID_ARGUMENT: Input text exceeds 256 characters. the bot should be able to answer to the user that the input is too long, even if this specific interaction is not specified in the execution model.

This can be handled either as static behavior defined in the xatkit-runtime component, or as an external execution model that could be weaved with the user defined one.

The first solution is probably easier to implement as a first approach, because dealing with exceptions from the execution model is not easy, and the model doesn't know the platform to target, so additional abstraction mechanisms are probably required.

In any case this requires to implement a set of methods to retrieve which are the platforms currently used by Xatkit, and select the one we want to use from Java code. This may be a bit tricky for bots running over multiple messaging platforms.

Submitted by @jcabot