void updateExpiry(long expiresAtMs) {
    ttlUpdated = true;
    expiresAt = expiresAtMs;
  }
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