Throw Exception when too many ids are generated in the same millisecond

[ssippe]

Firstly, thanks for putting this library together.

Based on my reading of the code (not testing), I think that it should throw an exception if AddOne is not able to increment anything here

The spec states "If, in the extremely unlikely event that, you manage to generate more than 280 ULIDs within the same millisecond, or cause the random component to overflow with less, the generation will fail.".

PR here #8