include key in error message from getField functions

[crcornwell]

What does this pull request do?

Extends the error message returned in the Left result of getField and getFieldOptional. This will take the message coming from decodeJson and prepend it with a helpful message that includes the specific key that failed to decode (e.g. Value is not a String will become Failed to decode 'foo': Value is not a String).

How should this be manually tested?

All tests are still passing, and this doesn't change the API in any way. To manually test you could create a Json value with a type that doesn't match what it should and see the new error on decode.

Other Notes:

This is a very simple change that will go a long way toward making the decoding error messages more useful, however a more robust solution akin to Aeson Better Errors would be ideal. For now though, I think this should be good.

[thomashoneyman]

Thanks @crcornwell! I think this is a useful addition to the library. Just two questions:

• Would you mind providing a test that should fail and output the correct error message?
• Can you think of other places it'd be possible to provide a more useful error message without changing the library's API? For example, could we note the index that failed when accessing an array?

[thomashoneyman]

We should make explicit exports for the module so we don't expose this function

[crcornwell]

@thomashoneyman sure, I can add a test. As far as outputting the array index in the error message I think that should be possible using something like traverseWithIndex, I can look into it.

[thomashoneyman]

If it's possible to do it'd be fantastic to add that into this PR at the same time. If not (or if it would take significant changes) then your proposal here is already useful.

[thomashoneyman]

Looks great! Glad to see both the array index and object key receiving clearer errors. Thanks for adding those extra tests to cover the new error messages, too.