Do not create system errors on duplicate credentials when linking oidc providers

[aeneasr]

Describe the bug

Currently, linking a oidc profile with an account where the oidc profile has already been linked to another account will result in a system error.

Reproducing the bug

1. Sign up a new identity using e.g. GitHub (idA)
2. Sign up another identity using e.g. password (idB) and go to settings page
3. Link the already used GitHub profile from idA to idB

Expected behavior

This should not result in a system error but instead in a validation error.

Environment

• Version: all

Additional context

Add any other context about the problem here.

[lmcl90]

It tried to create a duplicated identifier, not a credential.