Error messages

* Fixes https://github.com/giorgiosironi/eris/issues/36 by providing a way for the user to explicitate the input
* Starts the first shot at https://trello.com/c/3OeQFdkx/32-an-user-can-run-a-test-and-see-what-was-the-first-failed-example-and-the-following-shrinking-process as the whole process is very large, but at least the original failing input and the shrinked one are easy to show.

I will also look into an alternative way of passing parameters, different from environment variables.