new label: diagnostic quality

Hi devs, I've just created a new label in GitLab, called "diagnostic quality". (Diagnostics = errors ∪ warnings.) I intend for this label to be used when the report is about the quality of an error message or warning. That is, the existing message is not wrong, per se, but it's somehow not as helpful as it can be to users. I think it's helpful to separate out such issues from those describing error messages that are just wrong. In particular, if you spot a ticket arising from https://github.com/haskell/error-messages, you may want to give it the "diagnostic quality" label. If you disagree with the choice to make this label, please push back! Thanks, Richard