Refine error scopes

[m-bock]

Consider the following sample ADT:

data Sample
   = Foo
   | Bar Int String
   | Baz

and a wrongly encoded JSON:

{
  "tag" : "Bar",
  "value": [1, true]
}

It should produce an error similar to:

(Named "Sample" (Named "at case `Bar`" (AtIndex 1 (TypeMismatch "...."))))

Looking at it now, ideally JsonDecodeError would have an additional constructor AtCase.

@garyb Would you be up for extending JsonDecodeError?

[wclr]

@m-bock
Well as this is JSON related error, and there is no such thing as Case in JSON.

Note that new Codec.JSON does not define a special structured error type for errors. It uses Path pointer which has (AtKey, AtIndex) constructors, and a message for the context (which is Named in this lib).

[m-bock]

@wclr True, you're right!

[wclr]

Probably (AtKey "value"... also should be added here:
(Named "Sample" (Named "case Bar" (AtKey "value" (AtIndex 1 (TypeMismatch "....")))))