JSON-RPC errors are sent as string instead of object, not following JSON-RPC spec

[craigraw]

This issue is essentially a duplicate of romanz/electrs#313, which was fixed in romanz/electrs#390, romanz/electrs#398 and romanz/electrs#421 (as far as I can see - branching makes it a bit confusing).

Errors are returned as a string instead of an object. Here is an example of a batched response error:

[{"error":"verbose transactions are currently unsupported","id":204,"jsonrpc":"2.0"}]

which should be returned as

[{"error":{"code":-32601,"message":"verbose transactions are currently unsupported"},"id":204,"jsonrpc":"2.0"}]

following the JSON-RPC 2.0 spec.

Unfortunately this is a blocker for upgrading to batched JSON-RPC support on Sparrow currently.