Use flatMap in jsonToBinaryBase64

[lenguyenthanh]

This helps return better messaging in case of error.

Here are examples of (shorten) stack trace:

before:

[error] java.util.NoSuchElementException: None.get
[error]         at scala.None$.get(Option.scala:627)
[error]         at scala.None$.get(Option.scala:626)
[error]         at mongo4cats.circe.CirceJsonMapper$.jsonToBinaryBase64(CirceJsonMapper.scala:107)
[error]         at mongo4cats.circe.MongoJsonCodecs.$init$$

after:

[error] mongo4cats.errors$MongoJsonParsingException: DecodingFailure at ._id: "eCz830PN" is not a valid binary object
[error]         at mongo4cats.errors$MongoJsonParsingException$.apply(errors.scala:23)
[error]         at mongo4cats.circe.MongoJsonCodecs.mongo4cats$circe$MongoJsonCodecs$$anon$1$$_$get$$anonfun$2$$anonfun$1$$anonfun$1(MongoJsonCodecs.scala:99)
[error]         at scala.util.Either$LeftProjection.map(Either.scala:622)
[error]         at mongo4cats.circe.MongoJsonCodecs$$anon$1.get$$anonfun$2$$anonfun$1(MongoJsonCodecs.scala:99)
[error]         at scala.util.Either.flatMap(Either.scala:360)
[error]         at mongo4cats.circe.MongoJsonCodecs$$anon$1.get$$anonfun$2(MongoJsonCodecs.scala:99)