return NotFound on 404 errors

This changes raises docker.errors.NotFound on 404 errors.  This gives
client code the ability to differentiate between "an image does not
exist" and "you are using the api incorrectly".

This inherits from docker.errors.APIError so it will not affect any
existing code.

docker/clientbase.py

@@ -99,6 +99,8 @@ class ClientBase(requests.Session):
         try:
             response.raise_for_status()
         except requests.exceptions.HTTPError as e:
+            if e.response.status_code == 404:
+                raise errors.NotFound(e, response, explanation=explanation)
             raise errors.APIError(e, response, explanation=explanation)
 
     def _result(self, response, json=False, binary=False):

docker/errors.py

@@ -53,6 +53,10 @@ class DockerException(Exception):
     pass
 
 
+class NotFound(APIError):
+    pass
+
+
 class InvalidVersion(DockerException):
     pass