/auth/register error response

[mmahmoodictbd]

When email already exists in the system, API throws following with HTTP status code 500:
{ "email": [ "has already been taken" ] }

Response code should be 400 bad request.
Simplified API response would help mobile app. Proposed format:
{ "validationMessage": "Email already exist, try login." }

[imtiaz-emu]

@mmahmoodictbd PR created. Is the login error messages are okay? Or should I change that too?