Lemmy: Wrong error message when signing up with an email address that is already in use

[Nutomic]

to reproduce:

1. create an account with email
2. logout
3. create a second account, with a different username and the same email

result: error message "the user already exists"

[dessalines]

This one's a little trickier than others, because its a sql uniqueness constraint failing, and I have to hope that it tells me which constraint failed. I think its doable tho.