Improve error output. "Corrupt XZ stream error" returned for files with multi-streams

[Paul-E]

When reading a file with multiple streams, it is necessary to use new_multi_decoder (introduced in #31). Using XzDecoder::new on a multi-stream will result in an error like below:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "corrupt xz stream" }', src/main.rs:13:14

This message could be improved. Perhaps it should read "Corrupt xz stream, or multi-stream. Use new_multi_decoder if this is a multi-stream".

Additionally, is there a way to determine if something is a multi-stream before creating a decoder?

[alexcrichton]

Seems reasonable to me to improve!