Clean TypeScript Result type instead of throwing for the Frontend

[zupzup]

Come up with a nice Result<T> { success: T?, error: ErrorValue? } type that's returned from the API instead of promises that can throw.

The goal is to never throw, but rather have an error type and a success type.

Also, the ErrorValue should be an enum with all the possible error values for this call - if possible, make this more fine-grained (as fine-grained as possible, with the per-call-errors as the optimal. If it's not possible currently due to the effort, then create a future improvement ticket for this.