[Pig Wiki] Update of "PigDeveloperCookbook" by OlgaN

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http://wiki.apache.org/pig/PigDeveloperCookbook

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   * For `User Errors`, throw `ParseException` that contains meaningful message. For batch and interactive processing, catch the exception in Grunt and log the exception message to stderr. The same can be done with the developer in the embedded case. In debug mode, log the exception stack into the client side log.
   * For `Internal Errors`, throw `RuntimeException` or its derivation. Catch the exception in main, log it, including the stack, to the client side log. Write failure message to stderr pointing to the log file.
-  * For `Frontend Errors`, throw newly created `FrontendException`. Catch the exception in main, log it, including the stack, to the client side log. Write failure message to stderr pointing to the log file. In the future, we might find it useful to further subdivide this errors in which case we might subclass the `FrontendException` class. 
+  * For `Frontend Errors`, throw newly created `FrontendException` that is a subclass of RuntimeException. Catch the exception in main, log it, including the stack, to the client side log. Write failure message to stderr pointing to the log file. In the future, we might find it useful to further subdivide this errors in which case we might subclass the `FrontendException` class. 
   * For `Backend Errors` there will need to be backend specific way to get the error from the backend to the frontend. Once this is done, log the error into client side log file and throw `ExecuteException`. Catch the exception in main, log it, including the stack, to the client side log. Write failure message to stderr pointing to the log file.