Error handling guide

Now that we've done `fail` -> `panic`, I feel bringing back the error handling guide is a good idea. We had one long ago, but it was removed when conditions were removed.

This doesn't cover the new FromError stuff, but I feel like it's already useful in this state, so I'm sending this PR now.
I think we should perhaps make it clear that it is an int that we're trying to convert given that FromStr is a generic trait. Perhaps:

```rust
let n: uint = from_str("hello5world");
```