Mutation test should be explicit about the expected error

[pgrange]

Why

Our mutation tests check that, given an invalid transaction, the validator would fail. But if we just make a totally wrong mutation that creates a transaction which wouldn't make any sense, then the validator would fail but for the wrong reason and this limit the values of our tests.

We'd like to have each mutation test be explicit about the expected error from the validation script so that we are confident that our validator failed for the write reason.

What

Now we can associate, with any mutation, an optional expected error message. So let's update all the mutation to set an expected error message and ensure they're all failing for the good reason.

If it makes sense, we might start introducing some error messages systematic management, like error code or something. At least something which reduce plain string messages duplication between tests and validator code.