inherit from StandardError instead of exception

Background
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`Exception` is normally reserved for segfaults and other errors outside of the runtime. Inheriting form `Exception` can potentially interfere with things like SystemExit and interrupts. 

This changes the inheritance of `PlivoError` from `Exception` to `StandardError`.

 [Excellent discussion of this here] (http://www.skorks.com/2009/09/ruby-exceptions-and-exception-handling/).