Generate simpler operation error types for the server (#804)

Currently, operation error types are designed to be used by clients.
Clients need to cope with a large scale of possible errors, including
unmodeled errors and errors coming from different layers of the stack.
On the other hand, all operation server errors should be related to
business logic; we want all of them to be correctly modeled and have
compilation fail if any of them is not properly handled.

This commit revamps error generation by the `rust-server-codegen`
plugin, generating a simple Rust enum type for each fallible operation,
holding one variant per modeled error.

Closes #780.