Implement "Is()" For Error Type

[tri-adam]

We should implement the Is() method introduced in Go 1.13 for the jsonresp.Error type in order to make it more usable. I propose the implementation takes the approach similar to the example in the Working with Errors in Go 1.13 blog post, comparing non-zero fields.

This would allow succinct code such as the following:

if errors.Is(err, &jsonresp.Error{Code: http.StatusUnauthorized}) {
	// Do some stuff...
}