fix: better error msg for X-token registration

[EmileRolley]

This PR fix the first point suggered by @erikmd in the issue #405.

[erikmd]

Thanks, will merge as soon as the CI terminates.

[erikmd]

BTW @EmileRolley regarding GitHub assignees, if you don't disagree I'd suggest we follow the same approach as that of the Coq dev. team… which is a bit standard IINM:

• On an issue, the assignee is the developer/maintainer that will address the issue (open a PR)
• On a PR, the assignee is the maintainer that will merge the PR (after the approval from the reviewer(s))

[EmileRolley]

BTW @EmileRolley regarding GitHub assignees, if you don't disagree I'd suggest we follow the same approach as that of the Coq dev. team… which is a bit standard IINM:

• On an issue, the assignee is the developer/maintainer that will address the issue (open a PR)
• On a PR, the assignee is the maintainer that will merge the PR (after the approval from the reviewer(s))

MB, I will update this.

[erikmd]

Just a minor comment: it seems GItHub did not recognized your two e-mails as associated to your account:

maybe because you did git commit --amend and not git commit --amend --reset-author? cf. this screenshot from gitk:

anyway I guess it's a very minor issue! and it might suffice for you to keep this commit as is, and add the other e-mail address here? https://github.com/settings/emails

[EmileRolley]

maybe because you did git commit --amend and not git commit --amend --reset-author?

Indeed 😅. It's fixed.