Implement Display for Error and InvalidChunkSize

[17dec]

So that the errors can be passed to any macro/function that expects the Display trait.

Ideally these error types would also implement the Error trait, but that would require the std crate.

[coveralls]

Coverage remained the same at 97.368% when pulling f58d6d8 on 17dec:master into 2e64a82 on seanmonstar:master.

[seanmonstar]

Thanks!