Update unmarshal function to wrap ErrJSONUnmarshal. Requires at least Go 1.13.

[ewohltman]

Update unmarshal function to wrap ErrJSONUnmarshal and return specific error message.

Requires at least Go 1.13.

Resolves #923

[jolheiser]

LGTM, this seems nice for better troubleshooting and more accurate error responses. 👍

[CarsonHoffman]

Just for future reference: I considered whether it might make more sense to go all the way and expose the underlying JSON error via wrapping rather than just including its error text, but the following passage from Working with Errors in Go 1.13 is enlightening here:

It’s important to remember that whether you wrap or not, the error text will be the same. A person trying to understand the error will have the same information either way; the choice to wrap is about whether to give programs additional information so they can make more informed decisions, or to withhold that information to preserve an abstraction layer.

It doesn't seem to me like a program using an API wrapper would have any particular course of action based on a transit error, and thus this solution seems reasonable to me. Perhaps someone looking to extract that information in the future will see this—my point of putting my reasoning here—and it might be reasonable to revisit this in the future, but this is a good change now, so thank you for the change!