remove dead inverse method, add write tests, update read tests, add error enums used by tests

src/util/BaseRestClient.ts

@@ -131,8 +131,7 @@ export default abstract class BaseRestClient {
       await this.syncTime();
     }
 
-    const signedRequest = await this.signRequest(params);
-    return signedRequest;
+    return this.signRequest(params);
   }
 
   /**

src/util/enum.ts

@@ -0,0 +1,12 @@
+export const API_ERROR_CODE = {
+  ORDER_NOT_FOUND_OR_TOO_LATE: 20001,
+  CANNOT_SET_TRADING_STOP_FOR_ZERO_POS: 30024,
+  /** Seen when placing an order */
+  INSUFFICIENT_BALANCE_FOR_ORDER_COST: 30031,
+  /** Seen if a conditional order is too large */
+  INSUFFICIENT_BALANCE: 30042,
+  /** E.g. trying to change position margin while on cross */
+  POSITION_IS_CROSS_MARGIN: 30056,
+  RISK_LIMIT_NOT_EXISTS: 30090,
+  SAME_SLTP_MODE: 37002,
+} as const;