500 error when updating username to an already used username

[bethanyr]

Describe the bug
When I use the /PUT User api - and update a username to an already taken username, it returns a 500 Internal Server error

To Reproduce
Steps to reproduce the behavior:

1. Go to 'http://systers-mentorship-dev.eu-central-1.elasticbeanstalk.com/'
2. Click on 'PUT /user' Swagger entry
3. Enter your Bearer Token, and in the payload - include: "username": "isabel" - which is the name of an already created user
4. See error

Expected behavior
return a more descriptive error message than: 500

{
  "message": "Internal Server Error"
}

In the register user api call - if you use an existing username - you get a 400 error with this message:

{
  "message": "A user with that username already exists"
}

That might be better than returning a 500 Internal Server Error

Screenshots
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Desktop (please complete the following information):
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Smartphone (please complete the following information):
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Additional context
n/a

[isabelcosta]

Thank you @bethanyr for reporting this bug!
I just tested this now and it happens to me too.
I'll create an issue to solve this.