Better node errors using error messages in output types

[RunDevelopment]

This PR adds the ability for output types to return error messages. This is useful for complex types where never_reason alone isn't enough. Since never_reason is only one message, we previously weren't able to communicate the precise reason for the error, even though the type system knows the reason. I fixed this by allowing output types to return not only their type, but also an optional Error { message: string } type. This allows us to define relevant error messages, instead of just listing reasons why the node could be invalid.

In actual output type code, this looks like this (Alpha Matting node):

if fg <= bg {
    error("The foreground threshold must be greater than the background threshold.")
} else if bool::or(image.width != trimap.width, image.height != trimap.height) {
    error("The image and trimap must have the same size.")
} else {
    Image { width: image.width, height: image.height }
}

Note that this PR doesn't remove never_reason. never_reason is just simpler to use in many cases, and it still provides a useful fallback/default for error messages. I think of the new error("...") as an addition, not as a replacement.

[joeyballentine]

LGTM