This exception only exists for backwards compatibility with PHP versions
older than 5.3.0.
See https://www.php.net/manual/en/exception.getprevious.php
This marks PhutilProxyException as deprecated, as there is no need to use
it anymore. Removing it outright would break existing code.
The relevant methods of PhutilErrorHandler have also been marked for
deprecation, and simplified to always call Exception::getPrevious.