Consistency of error message when deserializing null to value vs. reference type

[cmeeren]

Deserializing null to a non-option-wrapped type produces a different error message depending on whether the target type is a value or reference type:

let options = JsonSerializerOptions()
options.Converters.Add(JsonFSharpConverter())
let deserialize<'a> (str: string) = JsonSerializer.Deserialize<'a>(str, options)

type A = { A: int; B: string }

deserialize<A> """{ "A": 2, "B": null }"""
// System.Text.Json.JsonException: A.B was expected to be of type String, but was null.

deserialize<A> """{ "A": null, "B": "a" }"""
// System.Text.Json.JsonException: The JSON value could not be converted to System.Int32. Path: $ | LineNumber: 0 | BytePositionInLine: 4. Path: $ | LineNumber: 0 | BytePositionInLine: 6. ---> System.InvalidOperationException: Cannot get the value of a token type 'Null' as a number.

Perhaps this could be made consistent? Something like:

System.Text.Json.JsonException: Null is not allowed here. <path, line number, etc.>

(Of course, the path/line number/position is wrong, as detailed in #37).

I don't care that much about this issue, I just noticed it and wanted to mention it.