retrofit-mock/src/main/java/retrofit2/mock/NetworkBehavior.java
public Response<?> createErrorResponse() {
    Response<?> call;
    try {
      call = errorFactory.call();
    } catch (Exception e) {
      throw new IllegalStateException("Error factory threw an exception.", e);
    }
    if (call == null) {
      throw new IllegalStateException("Error factory returned null.");
    }
    if (call.isSuccessful()) {
      throw new IllegalStateException("Error factory returned successful response.");
    }
    return call;
  }
public Response<?> createErrorResponse() {
    Response<?> call;
    try {
      call = errorFactory.call();
    } catch (Exception e) {
      throw new IllegalStateException("Error factory threw an exception.", e);
    }
    if (call == null) {
      throw new IllegalStateException("Error factory returned null.");
    }
    if (call.isSuccessful()) {
      throw new IllegalStateException("Error factory returned successful response.");
    }
    return call;
  }
The HTTP error to be used when an error is triggered.