Breaking Change: NotImplementedException is UnimplementedError

More exception class names are changing, this time NotImplementedException is changing to UnimplementedError.

Lasse Nielsen writes:

We have changed the name of NotImplementedException to UnimplementedError.
For a short while, the old name still works, and the constructor is still const, but that will all go away. 

We will likely change some (or all, time will tell) of the thrown UnimplementedError to UnsupportedError for the cases where we don't ever intend for the class to support that operation.

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