Merge #5858

Merge #5858

5858: Draft: Show reason for failed rename refactoring r=rherrmann a=rherrmann

Return an error with a meaningful message for requests to
`textDocument/rename` if the operation cannot be performed.
Pass errors, raised by rename handling code to the LSP runtime.

As a consequence, the VS Code client shows and logs the request
as if a server-side programming error occured.
Screenshot of a rename error showing in VS Code
![invalid-rename-ui](https://user-images.githubusercontent.com/607182/91059560-2c08a380-e62a-11ea-9297-f092db935a3b.png)

I would kindly ask to get feedback from the maintainers if they can spare the time:
* Is the general direction of the proposed changes acceptable?
* I'm new to Rust. The code feels clumsy and redundant, please suggest improvements
if you find the time for. E.g. is there a simple replacement for `RenameError`?
* Should presenting the error with proper severity (i.e. not as a programming error) be part of this change or in a followup change?

See https://github.com/rust-analyzer/rust-analyzer/issues/3981

Co-authored-by: Rüdiger Herrmann <ruediger.herrmann@gmx.de>