Error Identification in forms

Part of #1106 (closed) (read this issue for access to the full audit and recommendations).

Not 100% sure about this one, as the mentionned error are browser errors (based on the required attribute that is present on the fields). We'd probably need to contact the auditor to see if it's better to override the browser behaviour here and get rid of the required attr.

Audit section (Section 3.3.1):

When the form on page https://open.audio/login has been sent empty, a message pops up: ‘Fill out this field’. This is an instruction, not an error message. Also, the message is not fixed; when you scroll it is unclear to which field the message belonged.

A better message would be something along the lines of ‘The field Username has not been filled out’. Also, when you’ve filled in an incorrect email address, you see the message ‘Fill in an email address’. This also doesn’t describe what went wrong, namely that it has not been filled in correctly. See also the form on page https://open.audio/signup.