Proto: provide correct bounds for seed availability

Since this is a small MR, I've grouped here two, three independent changes.

One set (first two commits) represents small changes that were made while working on !4163 (closed), and which are however orthogonal to that MR. This addresses comment !4163 (comment 851230993).

EDIT: I have split the initial MR in two, the other one being !5875 (merged).

This MR is a small update that should have been made in !3738 (merged), but was not spotted. It's about providing a useful error message when RPCs ask for data depending on a seed that is not available. Without these changes one gets an error of the type:

Error:
  Storage error:
    Missing key '/cycle/5/random_seed'.

A unit test checks we get the expected error messages.

The first commit is independent, it was made in preparation for !4163 (closed).

Checklist

• [n/a] Document the interface of any function added or modified (see the coding guidelines)
• [n/a] 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