Handle communication failures cleanly

Expand exceptions to cover more failures cases. This adds
CommunicationFailure to represent any failures while attempting
to communicate with the remote endpoint. This also adds a new base
exception class BaseException which should be used for all non-HTTP
related failures.

Change-Id: Ie3e1d45c520d816a3f491a85fde94a6c4edf295e

glanceclient/exc.py

@@ -16,14 +16,25 @@
 import sys
 
 
-class CommandError(Exception):
+class BaseException(Exception):
+    """An error occurred."""
+    def __init__(self, message=None):
+        self.message = message
+
+    def __str__(self):
+        return self.message or self.__class__.__doc__
+
+
+class CommandError(BaseException):
     """Invalid usage of CLI"""
-    pass
 
 
-class InvalidEndpoint(ValueError):
-    """The provided endpoint could not be used"""
-    pass
+class InvalidEndpoint(BaseException):
+    """The provided endpoint is invalid."""
+
+
+class CommunicationError(BaseException):
+    """Unable to communicate with server."""
 
 
 class ClientException(Exception):