How to deal with values with errors

Nobody has claimed this yet.

How to deal with values that may have errors ?

Should we allow that a value has error directly in the QuantityWithUnit or should it be another type ?
```ts
export interface QuantityWithUnit {
value: number|ValueWithError;
unitText: string;
unitCode: string|URL;
}

export interface ValueWithError {
value: number;
sd: number;
}
```

What would be the consequence ? I guess that each time we reuse this kind of object we would have to throw if ever we would not deal with a ValueWithError

Start with the QuantityWithUnit and ValueWithError interface examples in the issue and review the three existing comments. Resolve whether errors belong in QuantityWithUnit or a separate type, and document the consequences for every consumer; the work is done when the representation and handling expectations are agreed.

Written by the indexing model from the issue text.