Blocked image generation should throw

[mbleigh]

Right now depending on the type of image generation block it will either throw or return a response without an image. There seems to be a pre-filter that throws, giving an error like:

400 Bad Request. {"error":{"code":400,"message":"Image generation failed with the following error: The prompt could not be submitted. This prompt contains sensitive words that violate Google's Responsible AI practices.

But there's a separate failure condition that does not throw but returns a response like:

{
  generatedImages: [
    {
      image: {},
      raiFilteredReason: "Unable to show generated images. All images were filtered out because they violated Vertex AI's usage guidelines. You will not be charged for blocked images. Try rephrasing the prompt. If you think this was an error, send feedback. Support codes: 29310472",
      safetyAttributes: {}
    }
  ]
}

I believe the latter should throw with the raiFilteredReason included in the error message. If the generation does not return any images, it is an error.