Show error if account already exists

[henriquecbuss]

What issue does this PR close

Closes #821

Changes Proposed ( a list of new changes introduced by this PR)

• Show error when account name already exists on register page (we already had this, but something broke it - likely Enhancement/forms #658)

How to test ( a list of instructions on how to test this PR)

• Try to register with an account name that already exists (for example, the one from the issue: JujutestePtb)
• See error

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[lucca65]

working!

[juramos-2020]

@NeoVier
Esse teste é para ser feito na criação de uma nova conta correto?
Eu estou usando a comunidade CCON pois a da Cambiatus não possui o link de criação de uma conta

"Ainda não tem uma conta? Crie agora"

O teste pode ser feito usando essa comunidade?

[juramos-2020]

A mensagem de erro deve aparecer no campo "nome do usuário" correto?

[henriquecbuss]

Isso mesmo, Ju. Ainda não tinha sincronizado no staging, mas agora já pode testar no link que vc mandou (https://ccon.staging.cambiatus.io/register)

[juramos-2020]

Prontinho @NeoVier tudo funcionando conforme o esperado. 👌✅