Apply rustfmt to exercises

[PaulXiCao]

The exercises do not conform to the current rustfmt standard, e.g. error-handling exercise. Maybe you can discuss if adding a ci job that checks for rustfmt changes makes sense here?

[djmitche]

That makes a lot of sense! We deviate from rustfmt in examples in a few cases, just to keep them short, but the exercises should be properly formatted.