Improve "Could not find Cargo.toml" error handling

[alexheretic]

Rls needs a root or parent Cargo.toml to work with multiple crates.

Rls sends down an error describing the issue now which is better than it was (#49, #57) but is still a bit of a messy error the first time you see it (#68).

Can we do better?

• Offer a hint message with a fix button: Button adds a root Cargo.toml with auto-detected all child folders with Cargo.toml files
• Instead offer just a hint with a link to https://github.com/mehcode/atom-ide-rust#multi-crate-projects

Note: Detecting that the error has happened is a bit of a pain, until maybe rust-lang/rls#723 is implemented.