Improve the error feedback on input errors

Is your feature request related to a problem? Please describe.
When user fails to fill a form correctly, most of the times they will see an error such as "is invalid" next to the field they tried to fill. This does not provide much feedback for the user on what they can do to resolve the issue.

Describe the solution you'd like
I would like the field errors to specifically report what was wrong. E.g. "you can only use letters a-z" or something along those lines, depending on the context.

Describe alternatives you've considered
Writing an endless amount of instructions that the users never read anyways.

Additional context
This particular issue we have been struggling with is regarding the "nickname" field in the registration field of the current 0.27 version. I know in the upcoming versions, the field will be gone but this could be still applied to other such fields that the users typically mistype. One of such field is the "URL slug" fields in the admin panel that the users fail to understand or to fill out correctly.

Does this issue could impact on users private data?
No

Funded by
N/A