[designspace] Can't generate variable font fixed axis subset

I have a large family with a weight and optical size axis. Currently trying to use designspace 5 to split the family into multiple variable fonts: specific values for weight with the whole optical size axis.

```xml

...

```

I get a `InvalidDesignSpaceData("No default source.")` error if I include a stop on the weight axis that doesn't have a corresponding source: i.e.`uservalue="100"` works, as it has a corresponding source, but `uservalue="200"` doesn't — it doesn't have a corresponding source in the sources for the weight axis.

This is unexpected—I was under the impressing that you could slice to a specific value anywhere on an axis, which would be so very nice. This behavior isn't documented anywhere as far as I can find; is it the correct behavior?