Use human readable exception messages

Currently, the exceptions are nothing if converted to strings.
The doc strings are human reable and useful to users, so use them.
Before:

    $ python examples/connection.py
    ERROR: Exception raised:
    $

After:

    $ python examples/connection.py
    ERROR: Exception raised: Could not find requested endpoint in Service Catalog.
    $

Change-Id: I1300e411c93bdfb17975e4f61e925b5b6474434c

keystoneauth1/exceptions/auth.py

@@ -14,5 +14,4 @@ from keystoneauth1.exceptions import base
 
 
 class AuthorizationFailure(base.ClientException):
-    """Cannot authorize API client."""
-    pass
+    message = "Cannot authorize API client."

keystoneauth1/exceptions/auth_plugins.py

@@ -14,11 +14,11 @@ from keystoneauth1.exceptions import base
 
 
 class AuthPluginException(base.ClientException):
-    """Something went wrong with auth plugins."""
+    message = "Something went wrong with auth plugins."
 
 
 class MissingAuthPlugin(AuthPluginException):
-    """An authenticated request is required but no plugin available."""
+    message = "An authenticated request is required but no plugin available."
 
 
 class NoMatchingPlugin(AuthPluginException):

keystoneauth1/exceptions/base.py

@@ -16,3 +16,12 @@ __all__ = ['ClientException']
 
 class ClientException(Exception):
     """The base exception for everything to do with clients."""
+    message = None
+
+    def __init__(self, message=None):
+        if not message:
+            if self.message:
+                message = self.message
+            else:
+                message = self.__class__.__name__
+        super(Exception, self).__init__(message)

keystoneauth1/exceptions/catalog.py

@@ -19,14 +19,12 @@ __all__ = ['CatalogException',
 
 
 class CatalogException(base.ClientException):
-    """Something is rotten in Service Catalog."""
+    message = "Something is rotten in Service Catalog."
 
 
 class EndpointNotFound(CatalogException):
-    """Could not find requested endpoint in Service Catalog."""
-    pass
+    message = "Could not find requested endpoint in Service Catalog."
 
 
 class EmptyCatalog(EndpointNotFound):
-    """The service catalog is empty."""
-    pass
+    message = "The service catalog is empty."

keystoneauth1/exceptions/connection.py

@@ -26,23 +26,19 @@ class RetriableConnectionFailure(Exception):
 
 
 class ConnectionError(base.ClientException):
-    """Cannot connect to API service."""
-    pass
+    message = "Cannot connect to API service."
 
 
 class ConnectTimeout(ConnectionError, RetriableConnectionFailure):
-    """Timed out connecting to service"""
-    pass
+    message = "Timed out connecting to service"
 
 
 class ConnectFailure(ConnectionError, RetriableConnectionFailure):
-    """A retryable connection failure."""
-    pass
+    message = "A retryable connection failure."
 
 
 class SSLError(ConnectionError):
-    """An SSL error occurred."""
-    pass
+    message = "An SSL error occurred."
 
 
 class UnknownConnectionError(ConnectionError):

keystoneauth1/exceptions/discovery.py

@@ -18,8 +18,8 @@ __all__ = ['DiscoveryFailure',
 
 
 class DiscoveryFailure(base.ClientException):
-    """Discovery of client versions failed."""
+    message = "Discovery of client versions failed."
 
 
 class VersionNotAvailable(DiscoveryFailure):
-    """Discovery failed as the version you requested is not available."""
+    message = "Discovery failed as the version you requested is not available."

keystoneauth1/exceptions/response.py

@@ -18,7 +18,7 @@ __all__ = ['InvalidResponse']
 
 
 class InvalidResponse(base.ClientException):
-    """The response from the server is not valid for this request."""
+    message = "The response from the server is not valid for this request."
 
     def __init__(self, response):
         super(InvalidResponse, self).__init__()