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.
Resolves https://github.com/rust-analyzer/rust-analyzer/issues/3981
Previously, "find all references" on a variant field wouldn't find any
references outside the defining module. This is because variant fields
were incorrectly assumed to be private, like struct fields without
explicit visibility, but they actually inherit the enum's visibility.
We might want to provide more efficient impls for check if usages
exist, limiting the search, filtering and cancellation, so let's
violate YAGNI a bit here.
