This removes `PerformError`, which was needed when we still had
polylith.
This removes quite a bunch of
```go
if err != nil {
return err
}
if err := res.Error; err != nil {
return err.JSONResponse()
}
```
Hopefully can be read commit by commit. [skip ci]
This adds tests for `/profile`.
Also, as a first change in this regard, refactors the methods defined on
the `UserInternalAPI` to not use structs as the request/response
parameters.
