Polluting the global space

[This file](https://github.com/plivo/plivo-ruby/blob/3a251883038331260ea88653d0b58cc332df75b3/lib/plivo/exceptions.rb) pollutes the global space. Can you please properly scope these classes to the `Plivo` namespace?

E.g.
<img width="396" alt="Screen Shot 2022-12-19 at 5 01 12 PM" src="https://user-images.githubusercontent.com/14316723/208557472-1c95cc25-7057-4ee5-acc5-829cacbbc5b5.png">

