Proto/Tests: assert errors explicitly in AI tests

This MR asserts errors explicitly in AI tests.

Manually testing the MR

The following tests were updated:

• test_scenario_slashing.ml
• test_scenario_slashing_stakers.ml
• test_scenario_deactivation.ml
• test_scenario_stake.ml
• test_scenario_base.ml

dune exec src/proto_alpha/lib_protocol/test/integration/main.exe -- --file test_scenario_slashing.ml

Checklist

• 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