Add retry_if_exception_message and complement

This is just a quick draft to get ideas on the table.

Open to suggestions on default logic, naming or copying this into your
own branch and opening a separate MR.

tenacity/retry.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 
 import abc
+import re
+from functools import wraps
 
 import six
 
@@ -86,7 +88,7 @@ def __call__(self, attempt):
         # always retry if no exception was raised
         if not attempt.failed:
             return True
-        return self.predicate(attempt.exception())
+        return self.predicate(str(attempt.exception()))
 
 
 class retry_if_result(retry_base):
@@ -111,6 +113,50 @@ def __call__(self, attempt):
             return not self.predicate(attempt.result())
 
 
+class retry_if_exception_message(retry_base):
+    """Retries if an exception message equals or matches."""
+
+    def __init__(self, message=None, match=None, require_exception=False):
+        if message and match:
+            raise TypeError("retry_if_exception_message() takes either 'message' or 'match', not both")
+        self.predicate = self._predicate_factory(message, match)
+        self.require_exception = require_exception
+
+    def _predicate_factory(self, message, match):
+        if message:
+            def message_fnc(exception_message):
+                return message == str(exception_message)
+            return message_fnc
+        elif match:
+            prog = re.compile(match)
+
+            def match_fnc(exception_message):
+                return prog.match(str(exception_message))
+            return match_fnc
+        else:
+            raise TypeError("retry_if_exception_message() missing 1 required argument 'message' or 'match'")
+
+    def __call__(self, attempt):
+        if not attempt.failed:
+            if self.require_exception:
+                raise RuntimeError("No exception to get errormessage from")
+            return True
+        return self.predicate(str(attempt.exception()))
+
+
+class retry_if_not_exception_message(retry_if_exception_message):
+    """Retries until an exception message equals or matches."""
+
+    def _predicate_factory(self, *args, **kwargs):
+        inverted_predicate = super(retry_if_not_exception_message, self)._predicate_factory(*args, **kwargs)
+
+        @wraps(inverted_predicate)
+        def predicate(*args_, **kwargs_):
+            return not inverted_predicate(*args_, **kwargs_)
+
+        return predicate
+
+
 class retry_any(retry_base):
     """Retries if any of the retries condition is valid."""