Clean up exception messages

In https://review.openstack.org/#/c/212670/ there was some
discussion about cleaning up these exception messages.  Here
it is.

Change-Id: I6bf1d7351f5fd90451f6d0d7c7e2575e584cbc62

keystoneauth1/exceptions/auth_plugins.py

@@ -14,7 +14,7 @@ from keystoneauth1.exceptions import base
 
 
 class AuthPluginException(base.ClientException):
-    message = "Something went wrong with auth plugins."
+    message = "Unknown error with authentication plugins."
 
 
 class MissingAuthPlugin(AuthPluginException):

keystoneauth1/exceptions/catalog.py

@@ -19,7 +19,7 @@ __all__ = ['CatalogException',
 
 
 class CatalogException(base.ClientException):
-    message = "Something is rotten in Service Catalog."
+    message = "Unknown error with service catalog."
 
 
 class EndpointNotFound(CatalogException):

keystoneauth1/exceptions/connection.py

@@ -30,11 +30,11 @@ class ConnectionError(base.ClientException):
 
 
 class ConnectTimeout(ConnectionError, RetriableConnectionFailure):
-    message = "Timed out connecting to service"
+    message = "Timed out connecting to service."
 
 
 class ConnectFailure(ConnectionError, RetriableConnectionFailure):
-    message = "A retryable connection failure."
+    message = "Connection failure that may be retried."
 
 
 class SSLError(ConnectionError):

keystoneauth1/exceptions/discovery.py

@@ -22,4 +22,4 @@ class DiscoveryFailure(base.ClientException):
 
 
 class VersionNotAvailable(DiscoveryFailure):
-    message = "Discovery failed as the version you requested is not available."
+    message = "Discovery failed. Requested version is not available."

keystoneauth1/exceptions/response.py

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