Makefile: add kaitai-struct-files-update target, improve check failure msg

Context

When kaitai check fail in the or locally, the echoed message gives no clue on how to fix the problem.

I added a target to only produce the kaitai descriptions, without the build.
It allows to do the generation without relying on tools that are not necessarily present on the developer machine.

This might be debatable if the generated description are not always correct. Comments welcome on that.

My take is that the devs will push the generated description and that the CI will ensure they pass the kaitai compiler.

Manually testing the MR

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