dagpool: PropagateError was not compatible with pickle

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

eventlet/dagpool.py

@@ -38,12 +38,19 @@ class PropagateError(Exception):
         the exception object raised by the greenthread
     """
     def __init__(self, key, exc):
+        # initialize base class with a reasonable string message
+        msg = "PropagateError({0}): {1}: {2}" \
+              .format(key, exc.__class__.__name__, exc)
+        super(PropagateError, self).__init__(msg)
+        self.msg = msg
+        # Unless we set args, this is unpickleable:
+        # https://bugs.python.org/issue1692335
+        self.args = (key, exc)
         self.key = key
         self.exc = exc
 
     def __str__(self):
-        return "PropagateError({0}): {1}: {2}" \
-               .format(self.key, self.exc.__class__.__name__, self.exc)
+        return self.msg
 
 
 class DAGPool(object):