Raise errors instead of returning null

[mattjohnsonpint]

If you pass a time zone that doesn't exist, most of the functions return null. Instead they should raise an error.

[mattjohnsonpint]

Additionally, as mentioned in the readme, the LocalToUtc and ConvertZone functions will return null if SkipOnSpringForwardGap = 0 and the local value falls into the gap. This should also raise an error instead of returning null, and the readme should be updated.