JSON API: fix up two existing warnings to be conformant.

Technically an API break, but nobody relies on these I hope!

Note that the feerates warning was buried inside the style object:
it should be top-level.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/libplugin-pay.c

@@ -1076,7 +1076,7 @@ static void payment_finished(struct payment *p)
 
 			if (result.leafstates != PAYMENT_STEP_SUCCESS)
 				json_add_string(
-				    ret, "warning",
+				    ret, "warning_partial_completion",
 				    "Some parts of the payment are not yet "
 				    "completed, but we have the confirmation "
 				    "from the recipient.");