Improve RPC errors Display

Example with new formatting:

  sendrawtransaction RPC error -22: TX decode failed. Make sure the tx has at least one input.

Instead of:

  sendrawtransaction RPC error -22: {"code":-22,"message":"TX decode failed. Make sure the tx has at least one input."}

src/daemon.rs

@@ -88,10 +88,13 @@ fn parse_jsonrpc_reply(mut reply: Value, method: &str, expected_id: u64) -> Resu
         if let Some(err) = reply_obj.get_mut("error") {
             if !err.is_null() {
                 if let Some(code) = parse_error_code(&err) {
+                    let msg = err["message"]
+                        .as_str()
+                        .map_or_else(|| err.to_string(), |s| s.to_string());
                     match code {
                         // RPC_IN_WARMUP -> retry by later reconnection
                         -28 => bail!(ErrorKind::Connection(err.to_string())),
-                        code => bail!(ErrorKind::RpcError(code, err.take(), method.to_string())),
+                        code => bail!(ErrorKind::RpcError(code, msg, method.to_string())),
                     }
                 }
             }

src/errors.rs

@@ -9,7 +9,7 @@ error_chain! {
             display("Connection error: {}", msg)
         }
 
-        RpcError(code: i64, error: serde_json::Value, method: String) {
+        RpcError(code: i64, error: String, method: String) {
             description("RPC error")
             display("{} RPC error {}: {}", method, code, error)
         }