Support pickling of exceptions

src/protean/exceptions.py

@@ -17,6 +17,9 @@ def __init__(self, messages, **kwargs):
     def __str__(self):
         return f"{dict(self.messages)}"
 
+    def __reduce__(self):
+        return (ProteanException, (self.messages,))
+
 
 class ConfigurationError(Exception):
     """Improper Configuration encountered like:

tests/test_exceptions.py

@@ -0,0 +1,12 @@
+import pickle
+
+from protean.exceptions import ObjectNotFoundError
+
+
+def test_pickling_of_exceptions():
+    exc = ObjectNotFoundError({"_entity": "foo"})
+
+    pickled_exc = pickle.dumps(exc)
+    unpickled_exc = pickle.loads(pickled_exc)
+
+    assert exc.messages == unpickled_exc.messages