NPE in KNXBinding for unsupported datapoints

This fixes the two NullPointerExceptions that have been thrown when an unsupported datapoint was used in the KNX binding of an item.

The first commit (369b52a) fixes the two NPEs. I made the code a little bit more verbose by introducing a nested "if" statement and a new local variable "dptTypeClass". This makes it clear that there could be a null value.

The second commit throws an exception (and thus a message during startup of openhab), if the item configuration contains an unsupported datapoint. This gives clear feedback that something is wrong with the configuration.