public void setErrorCodeForBlob(String blobId, ServerErrorCode errorCode) {
    if (errorCode == null) {
      errorCodeForBlobs.remove(blobId);
    } else {
      errorCodeForBlobs.put(blobId, errorCode);
    }
  }
Sets up this {@link MockServer} such that it returns the given {@code errorCode} for the given {@code blobId} for
get, ttl update and delete (not put).
@param blobId the blob id for which the error code must apply
@param errorCode the error code to apply
ambry-router/src/test/java/com/github/ambry/router/MockServer.java