feat: fromJson? derivations now say where a parsing error occurred

[EdAyers]

Consider the following structures

structure Test1 where
  hello : String
  cheese? : Option Nat
  deriving ToJson, FromJson, Repr

structure Test2 where
  jam: Test1
  deriving ToJson, FromJson, Repr

#eval fromJson? (α := Test2) <| Json.mkObj [("jam", Json.mkObj [("hello", 4)])]

Previously, the expression in #eval would return an error "String expected". However often these parsing errors can occur in very deeply nested structures, making it hard to discover where exactly a string was inspected.

This PR causes the fromJson? method to prepend the field where the failure occurred if parsing fails. So now the above example will yield "Test2.jam: Test1.hello: String expected", making it clear that the badly formed sub-json was in jam.hello.