[clangd/index/remote]NFC: Adapt code to newer grpc/protobuf versions

It seems newer grpc / protobuf versions renamed
`Status::error_message()` and `Status::error_code()` to `message()` and
`code()` to prepare for replacement with `absl::Status` with the same
names.

As far as I can tell the new names are already available in the
grpc-1.36 version mentioned in the `README` file.

Differential Revision: https://reviews.llvm.org/D144599