Bugfix/user 409 feedback

[tobias-wiese]

Description

Fixes issue #419

Reason for this PR

• see No error shown for adding a conflicting account #419

Changes in this PR

• add error handling for conflicting accounts

Type of change (remove all that don't apply)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Manually tested all use cases this could alter

Test Configuration:

• OS: Windows

• Browser: Firefox

• Frontend: (remove all that don't apply)

  • Development build

• Backend: (remove all that don't apply)

  • deployed development build

Checklist: (remove all that don't apply)

• I added corresponding E2E tests (especially for bugfixes)
• I have performed a self-review of my own code
• My changes generate no new linting warnings or console warnings
• I have updated the CHANGELOG.md to include any changes made in this PR (add WIP to the top, if there is none already)