Allow customization of the error message for missing element when decoding

Currently decoding a JSON that misses an element will result in a DecodingFailure with a message saying: "Attempt to decode value on failed cursor". This error cannot be customized other than by doing string matching on DecodingFailure.message. I would suggest to add a default parameter on decode and decodeAccumulating:

final def decode[A: Decoder](input: String, missingElementErrorMessage: String =
                             defaultMissingElementErrorMessage)

An alternative would be to encode that case in the type:

sealed trait DecodingFailure { def history: List[CursorOp] }
case class MissingElement(val history: List[CursorOp]) extends DecodingFailure
case class IncorrectValue( /* ... */ ) extends DecodingFailure

[exoego]

I opended a PR #1081 to implement this feature, but no response for 3 months. If it is unlikely to be implemented in circe, please close this issue as wontfix.

[knutwalker]

Maybe this might help; I did change the message by looking at the cursor history and trying to examine whether the failure was due to a missing field (which could be simplified a bit by using CursorOp.opsToPath): https://github.com/knutwalker/akka-stream-json/blob/b8bc752ef3ab2f76793c0db40bddc8a2bd5787df/support/stream-circe/src/main/scala/de/knutwalker/akka/stream/support/CirceStreamSupport.scala#L46-L73