Exception class hierarchies

All of PEAR packages exceptions must be descendant from PEAR_Exception. PEAR_Exception provides exception wrapping abilities, absent from the top level PHP Exception class, and needed to comply with the previous section requirements.

Aditionally, each PEAR package must provide a top level exception, named <Package_Name>_Exception. It is considered best practice that the package never throws Exceptions that aren't descendant from its top level exception.