Add exceptions for 500 and 503 HTTP status codes

Add glance.exc.InternalServerError and .ServiceUnavailable to
represent HTTP statuses 500 and 503. Users will definitely see
these from Glance, so let's be nice and map them to handy
exception classes.

Change-Id: I8e8fcda532455793ea4d0f08a23f7c92b68c186c

glanceclient/exc.py

@@ -90,6 +90,14 @@ class OverLimit(ClientException):
     message = "Over limit"
 
 
+class InternalServerError(ClientException):
+    """
+    HTTP 500 - Internal Server Error
+    """
+    http_status = 500
+    message = "Internal Server Error"
+
+
 # NotImplemented is a python keyword.
 class HTTPNotImplemented(ClientException):
     """
@@ -99,14 +107,23 @@ class HTTPNotImplemented(ClientException):
     message = "Not Implemented"
 
 
+class ServiceUnavailable(ClientException):
+    """
+    HTTP 503 - Service Unavailable
+    """
+    http_status = 503
+    message = "Service Unavailable"
+
+
 # In Python 2.4 Exception is old-style and thus doesn't have a __subclasses__()
 # so we can do this:
 #     _code_map = dict((c.http_status, c)
 #                      for c in ClientException.__subclasses__())
 #
 # Instead, we have to hardcode it:
-_code_map = dict((c.http_status, c) for c in [BadRequest, Unauthorized,
+_exc_list = [BadRequest, Unauthorized, Forbidden, NotFound, OverLimit,
-                   Forbidden, NotFound, OverLimit, HTTPNotImplemented])
+             InternalServerError, HTTPNotImplemented, ServiceUnavailable]
+_code_map = dict((c.http_status, c) for c in _exc_list)
 
 
 def from_response(response):