jsonrpc: extract error data from JSON RPC error

Inspired by 'spelunk' from ethers.js:
f97b92bbb1/packages/providers/src.ts/json-rpc-provider.ts (L25)

ethers/provider.nim

@@ -14,6 +14,7 @@ export blocktag
 type
   Provider* = ref object of RootObj
   ProviderError* = object of EthersError
+    data*: ?seq[byte]
   Subscription* = ref object of RootObj
   EventFilter* {.serialize.} = ref object of RootObj
     address*: Address

ethers/providers/jsonrpc/errors.nim

@@ -1,13 +1,30 @@
+import std/strutils
 import pkg/stew/byteutils
+import ../../basics
 import ../../provider
 import ./conversions
 
+{.push raises:[].}
+
 type JsonRpcProviderError* = object of ProviderError
 
-func new(_: type JsonRpcProviderError, json: JsonNode): ref JsonRpcProviderError =
+func extractErrorData(json: JsonNode): ?seq[byte] =
+  if json.kind == JObject:
+    if "message" in json and "data" in json:
+      let message = json{"message"}.getStr()
+      let hex = json{"data"}.getStr()
+      if "reverted" in message and hex.startsWith("0x"):
+        if data =? hexToSeqByte(hex).catch:
+          return some data
+    for key in json.keys:
+      if data =? extractErrorData(json{key}):
+        return some data
+
+func new*(_: type JsonRpcProviderError, json: JsonNode): ref JsonRpcProviderError =
   let error = (ref JsonRpcProviderError)()
   if "message" in json:
     error.msg = json{"message"}.getStr
+  error.data = extractErrorData(json)
   error
 
 proc raiseJsonRpcProviderError*(

testmodule/providers/jsonrpc/testErrors.nim

@@ -0,0 +1,27 @@
+import std/unittest
+import std/json
+import pkg/questionable
+import pkg/ethers/providers/jsonrpc/errors
+
+suite "JSON RPC errors":
+
+  test "converts JSON RPC error to Nim error":
+    let error = %*{ "message": "some error" }
+    check JsonRpcProviderError.new(error).msg == "some error"
+
+  test "converts error data to bytes":
+    let error = %*{
+      "message": "VM Exception: reverted with 'some error'",
+      "data": "0xabcd"
+    }
+    check JsonRpcProviderError.new(error).data == some @[0xab'u8, 0xcd'u8]
+
+  test "converts nested error data to bytes":
+    let error = %*{
+      "message": "VM Exception: reverted with 'some error'",
+      "data": {
+        "message": "VM Exception: reverted with 'some error'",
+        "data": "0xabcd"
+      }
+    }
+    check JsonRpcProviderError.new(error).data == some @[0xab'u8, 0xcd'u8]

testmodule/providers/testJsonRpc.nim

@@ -2,5 +2,6 @@ import ./jsonrpc/testJsonRpcProvider
 import ./jsonrpc/testJsonRpcSigner
 import ./jsonrpc/testJsonRpcSubscriptions
 import ./jsonrpc/testConversions
+import ./jsonrpc/testErrors
 
 {.warning[UnusedImport]:off.}