Duplicate account issues

[vcfxb]

On of the testers created an account using GitHub, despite already having an account linked to RPI CAS & Discord.
When trying to link RPI CAS to the account created with GitHub, they landed on this error page.

This error should be checked for and modified to avoid having a JSON error object in the user interface.
In the bigger picture, users should be able to delete their accounts (#15) or merge existing accounts. Both features are desirable, but merging accounts would be especially nice with regards to (#135).