auto merge of #12414 : DaGenix/rust/failing-iterator-wrappers, r=alexcrichton

Most IO related functions return an IoResult so that the caller can handle failure in whatever way is appropriate. However, the `lines`, `bytes`, and `chars` iterators all supress errors. This means that code that needs to handle errors can't use any of these iterators. All three of these iterators were updated to produce IoResults.

Fixes #12368

diff --cc src/libstd/io/mod.rs
Simple merge