Context

We want to get rid of the dependency on Legacy_monad_globals from lib_error_monad, for proto_alpha.

In order to do that, we need to explicitly open the correct error monads, so that we remove the dependency on the legacy module.

This is the necessary part for src/proto_alpha/.

Manually testing the MR

CI green

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR