Clarify Inlet documentation

[adayton1]

Can someone clarify the following statement from the documentation? What is meant by a runtime error? Are we talking an exception or something else?

"Accessing a value that was not provided by the user, or a default value, will result in a runtime error." (https://axom.readthedocs.io/en/develop/axom/inlet/docs/sphinx/simple_types.html#accessing)

I'm also confused about what happens if you have a container that is optional, but it has a field or container that is required. I assume that if the container is not specified in the user input, no error is thrown. But if the container is specified, then the required children must be specified or there will be an error.