mockNetworkImages should provide a way to test the errorBuilder from Image.network

[DanielFerrariR]

Details:
Sometimes, we need to provide an errorBuilder for Image.network, and, at least so far, I didn't see a way to test it without mocking httpClient, which is basically what mockNetworkImages already does.

Expected:
mockNetworkImages should have a new option to make the request fail as failed to fetch the image, so Image.Network can use the errorBuilder instead.

I would really appreciate it if someone had a better and simpler way to do it.