Provide more detailed TOML decoding errors

[chshersh]

Currently, when TOML decoding fails due to missing fields or mismatched values, the error doesn't specify the missing field:

Error parsing configuration at path Cargo.toml: [Decode Error]

The corresponding function also simply returns Option<Config> instead of providing more details

https://github.com/chshersh/tool-sync/blob/950b4028a619d33303548acd06056949dcec1a4a/src/config/toml.rs#L48-L64

It would provide better UX to add more details to the DecodeError constructor in the TomlError type by telling all errors that happened during decoding:

https://github.com/chshersh/tool-sync/blob/950b4028a619d33303548acd06056949dcec1a4a/src/config/toml.rs#L10-L14

Open questions:

• Does the toml crate provide better decoding than as_str() and friends? All such functions return Option<...> while it would be better to have Result<..., ...>
• Does Rust have the analogue of the Haskell's Validation type to combine multiple errors and which can be used with toml?

[DukeManh]

Hey @chshersh, I ran into the same issue trying your tool. I'd like help with this.

[chshersh]

@DukeManh Sure, go for it!