PEAR2\Exception is the standard exception class for PEAR2.
In addition to supporting the default signature of the built-in
Exception class, PEAR2\Exception also supports passing a
cause of an exception, which can be another exception object
or a PEAR2_MultiErrors object.  The error messages of the causes
are used when constructing the error message displayed when
converting PEAR2\Exception to a string.

PEAR2\Exception should always be extended by your own custom exception
class for the package.