Proto/SCORU: remove failsafe mechanism for finalize_inbox

Context

This MR moves the no_history functions outside of the error monad, which allows us to add messages to inbox without failing. The only remaining errors are storage related ones, which is already the case with all the other functions called in begin application, block finalization and migration.

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/pbt/main.exe -- -f 'Smart rollup inbox'