[JS] Unhelpful error from imagen models in vertexai plugin

[maxl0rd]

When imagen model fails to generate any predictions, the error is obscure instead of throwing a reasonable exception.

TypeError: Cannot read properties of undefined (reading 'map')
    at /workspace/node_modules/@genkit-ai/vertexai/lib/imagen.js:249:47

Expected behavior A clear and concise description of what you expected to happen.

As for Gemini models

throw new Error('No valid candidates returned.')

[devchas]

Streaming the image result largely resolved this issue for me.

[damsorian]

@devchas can you share an example of your workaround?