Avoid fail!s

[wycats]

Instead of failing, return a result from from_toml, so it can be reported properly. We're using rust-toml in Cargo, and this is a blocker for us 😦

[wycats]

FWIW, a pattern I have used (a stopgap until we get better Result support in Rust proper) is to stash an error reason on the decoder struct, and then check for it in the main decoder loop (to skip future decodings that will likely fail).