Validate required property

[elliotcmorris]

If we add a tag to the yaml schema required to each property, there is some validation we can do to be friendlier. Most directly, we can emit a better error message when the property is not set, so the recipient knows the error is not their fault, and knows who to go to to address it.

[feltech]

The example potential solution given is not possible since we switched to std::optional rather than exceptions.

A more breaking change would be to remove the std::optionalness of required properties, and throw an exception saying the value should have been set.

[feltech]

Carefully consider this in the context of established rules in our GUIDELINES

[feltech]

I'm going to close this, since it reverses current guidelines, and the proposed solution wouldn't work anyway.