Improve UX of sign up errors for the demo instance

It's not nice for users to see that [KratosError] part of the error message. We need to remove that part.

We can also make the error message more user-friendly in these 2 cases at least:

• Entered valued is not an email address
• Email address has already been used

added UX label

changed the description

changed milestone to %2.4

added weight::0.5 label

changed the description

assigned to @arie

created branch 124-improve-ux-of-sign-up-errors-for-the-demo-instance to address this issue

added Doing label

mentioned in merge request !104 (merged)

Done. I also fixed the form so you can submit by pressing enter from the email address field.

mentioned in commit ae19bbb1

closed with merge request !104 (merged)