CodeTaskFactory error on missing attributes does not contain element name

When using CodeTaskFactory, if a Reference element is missing the Include attribute, or a Using element is missing the Namespace attribute, an error is printed about an empty attribute value.

This error isn't great for a few reasons

1. It doesn't say which element the attribute is on
2. It says "The "Include" attribute has been set but is empty" even when it's *missing*
3. It's emitted at the point the task is _used_, not declared, so the message about the Include attribute is easily confused for referring to the Items passed to the task