Refactor E2E image loads

[rainest]

Problem Statement

As designed the image loader effectively uses errors for downstream control flow. This:

if b, err := loadimage.NewBuilder().WithImage(imageLoad); err == nil {
	addons = append(addons, b.Build())
}

handles the imageLoad == '' condition by relying on the builder returning an error, when it could have used a conditional on imageLoad's value.

Proposed Solution

Use a conditional instead, and fail on any error.