I usually place regex validation on the input forms. For example in Norway, mobile-phone numbers are 8 numbers. If 8 numbers are input, I do a quick get-request to check if that phone number is taken. If its not taken, I show a green checkmark, if its taken, I mark the field red.
Same with emails.

As for the objects returned, I only throw 401 when Authentication fails or token is invalid. I usually respond with 200 and
Success JSON
{
success: true,
data: {
id: 1,
name: 'Bill Gates'
}
}
Error JSON
{
success: false,
msg: 'E-mail already taken.'
}