handle name attribute in errors correctly

[EmileTrotignon]

I added better error messages previously, but they ignored the [@name "..."] attribute. Its now correctly taken into account.

I added a test case in errors.t :

  Fatal error: exception Ppx_deriving_json_runtime.Of_json_error(Json_error {|expected ["A"] or ["Foo"] or ["B", _] or ["C", _, _] or ["D", { _ }] or ["different_name"] but got ["B", 123, "booh"]|})

Before this patch this would have been :

  Fatal error: exception Ppx_deriving_json_runtime.Of_json_error(Json_error {|expected ["A"] or ["Foo"] or ["B", _] or ["C", _, _] or ["D", { _ }] or ["Different_name"] but got ["B", 123, "booh"]|})

[anmonteiro]

just a small nit, otherwise looks good.

Don't forget to snapshot the tests.

[anmonteiro]

Suggested change

	let name = c.pcd_name in
	let name =
	Option.value ~default:name
	(vcs_attr_json_name (Vcs_ctx_variant c))
	in
	let name =
	let name = c.pcd_name in
	Option.value ~default:name
	(vcs_attr_json_name (Vcs_ctx_variant c))
	in