"Invalid SSH key data" for SSH key with no comment is unclear

[Daniel Manrique]

How to reproduce:

- Try adding an SSH key with no comment:
  - ssh-keygen -f commentless-key -C ''
  - https://launchpad.net/%7E/+editsshkeys and add the generated (commentless-key.pub) key which only contains "ssh-rsa $BASE_64_PAYLOAD".

What should happen:
  - A nice error saying 'Invalid data: a comment is required' or something telling me what's actually needed.

What actually happens:
  - Unclear error: Invalid SSH key data: "the-stuff-I-tried-to-post"

This error comes from lib/lp/registry/model/person.py SSHKeySet.extract_ssh_key_components.

The same error is sent to SSO via the API so any improvement in the error message here would also benefit SSH key handling from SSO.

I guess clearly stating what the expected format of the key is (both in SSO and Launchpad) would be a cheap mitigation; I was also under the impression a public SSH key was entirely unambiguous so "Invalid data" should be enough, but then ran into this case where a comment-less SSH key is valid and usable but Launchpad dislikes it.

[Daniel Manrique (roadmr)]

https://bugs.launchpad.net/launchpad/+bug/60601 is relevant but not exactly a dupe; I'd be happy to just get a relevant error but still making the comment mandatory, while that one wants allowing comment-less keys.