Auto merge of #3794 - alexcrichton:better-errors, r=matklad

Improve TOML decoding error messages

Unfortunately while `#[serde(untagged)]` is precisely what we want in terms of
semantics it leaves a little to be desired in terms of error messages. This
commit updates to remove the usage of that attribute in favor of implementing
`Deserialize` directly, which is quite simple in these few cases.

Closes #3790