Send window/showMessage when starting for project with no Cargo.toml

[alexheretic]

When starting Rls pointing towards a directory without a Cargo.toml we're not providing any feedback that this isn't expected to work very well.

A simple example is having a parent folder as your project, with rust crates as sub-folders. A simple fix is to use a cargo workspace in the parent. However, currently no warning message is shown and instead just nothing happens rust-lang/atom-ide-rust#49.

The vscode client has it's own code to warn A Cargo.toml file must be at the root of the workspace in order to support all features.

Should we instead have Rls itself send a warning window/showMessage to the client to allow all clients to warn of this? Or perhaps is there a good reason to keep it as per-client functionality?