Previous PropagateError.__init__() failed to call Exception.__init__() at all.
Moreover, as described in https://bugs.python.org/issue1692335, setting
self.args is important for an Exception subclass with nonstandard constructor
arguments.
When an Exception subclass sets self.args, it makes the subclass pickleable --
but instead of str(instance) returning the string passed to
Exception.__init__(), it instead returns the string representation of
self.args. So capture the message string, and in addition to passing it to
Exception.__init__(), also override __str__() and return the same string.
https://github.com/eventlet/eventlet/pull/359
f572fc9ae2