Better error handling for malformed CZML

I'm just starting to generate my own CZML and I've found that the Packet parser can at times be less than helpful when you feed it bad data, as in [this simple example](https://sandcastle.cesium.com/?src=Hello%20World.html&label=Showcases&gist=13311b55ffaaff8ed890a4c203641c89).

It bombs out parsing the second of 3 entities because I fed it an enum value (`2`, number) instead of the key it was expecting (`FILL_AND_OUTLINE`, string). That's on me, of course, but it would be very nice to have *helpful* console output when this happens -- something along the lines of `invalid value "2" for property 'label.labelStyle'`. Current behavior is that it just attempts a property access on an undefined value and throws an uncaught exception. From the user perspective, it might be nice to recover from such bad values and either ignore that individual property, or at least just discard the one entity with the malformed property and attempt to continue processing all the other entities / packets in the data.

Reproduce the malformed enum case from the linked Sandcastle example and trace the Packet parser entry point that handles the invalid label.labelStyle value. Determine whether the expected behavior is a console error, skipping the property, or discarding the entity and continuing; the issue does not choose between these options. Done should include helpful invalid-value reporting and defined recovery behavior for the remaining packets.

Written by the indexing model from the issue text.