Specify pop-up notifications for already registered email addresses.

[RaqueAbarca]

Is your feature request related to a problem? Please describe:

When attempting to register but the email is already in use, the pop-up notification only states that the email doesn't meet the requirements.

Describe the solution you'd like:
I think it would be very useful and helpful for the notification to indicate that the email doesn't meet the requirements because it's already in use, for example: "This email is already registered. Would you like to log in?

[r-southworth]

It is considered a security issue to acknowledge already registered email addresses. However this is a bug in that expected behavior would be to simply allow the attempted registration and ignore it on the backend.