Add error info

bdchain/api/common.proto

@@ -7,6 +7,12 @@ option java_package = "bdchain.api.grpc";
 option java_outer_classname = "CommonProto";
 option java_multiple_files = true;
 
+message Error {
+    string code = 1; // One of a server-defined set of error codes.
+    string message = 2; // A human-readable representation of the error.
+    string target = 3; // The target of the error.
+}
+
 /* 事务类型 */
 enum TransactionType {
     RECORD = 0; // 通用数据记录