Add `serde_json` Error variant

[ValuedMammal]

I personally wanted to remove the .unwrap() and properly handle the error, though the options I had in mind:

(i) adding a new SerdeJson variant is a breaking change; we can do it as a follow-up for 0.13.0
(ii) mapping and wrapping it into either minreq/reqwest errors is only possible in minreq, so I thought it's better to keep it standard and use .unwrap_or_default() on both for now.

Originally posted by @oleonardolima on #159 (comment)