Consistently use `anyhow` in `ra_project_model`

`ra_project_model` crate does a lot of interraction with other processes (`cargo` and `rustc`). It is bound to fail in interesting ways, so we need to make sure that the error reporting is top-notch. 

We should switch it to the `anyhow` crate and use `.context` method proacticaly to make sure that all errors are reported with context. 