Add errors7 exercise to show full potential of try operator combined with From trait

[gdziadkiewicz]

Thanks for creating the Rustlings! I loved it!

While doing them I felt that the user might miss the full potential of the try operator due to the lack of exercise that shows how well custom error types work with it and From trait. This PR adds an exercise focused on that. I placed it after the first trait exercise to be sure that they understand what they are doing.

Only after creating the thing did I find out that I should have created an issue first - sorry for that!

If it goes in let's squash it.

[mamalieaz]

salam