services should never return transport level errors

[butonic]

The CS3 API adopted the grpc error codes from the google grpc status package. It also separates transport level errors from application level errors on purpose. This allows sending CS3 messages over protocols other than GRPC. To keep that seperation, the server side must always return nil, even though the code generation for go produces function signatures for rpcs with an error return property. That allows clients to clearly distinguish between transport level errors indicated by err != nil the error and application level errors by checking the status code.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[glpatcern]

I would merge this to master as well! @ishank011 ?