New example: Return JSON decode errors to a client (#268)

* Add JSON decode error example

* cargo fmt

* Use more appropriate response codes

* curl -i similar for all examples

* Add one more example of 422 response

* use to_string instead of empty format!

Cargo.toml

@@ -16,6 +16,7 @@ members = [
    "hello-world",
    "http-proxy",
    "json",
+   "json_decode_error",
    "json_error",
    "jsonrpc",
    "juniper",