Parse RPC error responses correctly.
=/ res=(unit [@t json])
((ot id+so result+some ~) a)
?^ res `[%result u.res]
+ ~| a
:+ ~ %error %- need
- ((ot id+so code+so message+so ~) a)
+ ((ot id+so error+(ot code+no message+so ~) ~) a)
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