Disallow extending Throwable

_[Original issue](https://code.google.com/p/error-prone/issues/detail?id=167) created by **eaftan@google.com** on 2013-08-01 at 12:19 AM_

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Java allows you to extend Throwable, but it's generally a bad idea. Most error handling code assumes that the only direct subtypes of Throwable are Exception and Error, and do not deal correctly with other subtypes.