adding notes about dealing with exceptions in the client

Change-Id: I661b10414e0281bc695236bb32f985b0ef5b7600

HACKING

@@ -75,3 +75,9 @@ Docstrings
   :returns: description of the return value
 
   """
+
+Exceptions
+----------
+
+When dealing with exceptions from underlying libraries, translate those
+exceptions to an instance or subclass of ClientException.

doc/source/using-api.rst

@@ -29,6 +29,10 @@ You create a valid ``keystoneclient.v2_0.client.Client`` object by passing
 authentication data to the constructor. Authentication and examples of common
 tasks are provided below.
 
+You can generally expect that when the client needs to propogate an exception
+it will raise an instance of subclass of
+``keystoneclient.exceptions.ClientException``
+
 Authenticating
 ==============