It would be useful to have a way to terminate a query with a custom error message. This would be particularly useful in the LINQ translator for the C# driver.

For example, LINQ has a method called `Single` that verifies that an array has a single element and returns that element. The query should terminate if the array is empty or has more than one element.

Assumingn the existence a `$error` operator to terminate the query, the following LINQ expression:

x.ArrayField.Single()

could be translated to something like the following MQL:

{ $switch :
    branches : [
        { case : { $eq : [{ $size : "$ArrayFIeld" }, 0] }, then : { $error : "array is empty" } },
        { case : { $gt : [{ $size : "$ArrayFIeld" }, 1] }, then : { $error : "array has more than one element" } }
    ],
    default; { $arrayElemAt : ["$ArrayField", 0] }
}