Return reason of why proof cant be generated (api change)

[rafal-ch]

⚠️ This should only be merged into the next, planned breaking release ⚠️ ⚠️ I'll keep it as draft to avoid accidental merge ⚠️

Linked Issues/PRs

Additional changes related to https://github.com/FuelLabs/fuel-core/issues/1394 Closes: https://github.com/FuelLabs/fuel-core/issues/2367

Description

Initially we wanted these changes, but eventually decided to revert them from the original PR, because even though they are not considered strictly breaking, they change the API.

Checklist

• [X] Breaking changes are clearly marked as such in the PR description and changelog

Before requesting review

• [X] I have reviewed the code myself