Add exceptions that return more information about client & server errors
 - any 400/500 response will generate an exception instead of returning null
 - any non 400/500 response will return a response
 - should 300's be considered?
Simplify throws clause
remove import
`UnauthorizedException` now extends `ClientErrorException`
Merge pull request #5 from GlobalTechnology/add-exceptions

Add exceptions that return more information about client & server errors