Let's have the new exceptions subclass basic exception types

tests/unit/test_cassettes.py

@@ -1,7 +1,8 @@
 import pytest
 import yaml
 import mock
-from vcr.cassette import Cassette, UnhandledHTTPRequestError
+from vcr.cassette import Cassette
+from vcr.errors import UnhandledHTTPRequestError
 
 
 def test_cassette_load(tmpdir):

vcr/cassette.py

@@ -13,10 +13,7 @@ from .patch import install, reset
 from .persist import load_cassette, save_cassette
 from .serializers import yamlserializer
 from .matchers import requests_match, url, method
-
-
-class UnhandledHTTPRequestError(Exception):
-    pass
+from .errors import UnhandledHTTPRequestError
 
 
 class Cassette(ContextDecorator):

vcr/errors.py

@@ -0,0 +1,10 @@
+class CannotOverwriteExistingCassetteException(Exception):
+    pass
+
+
+class UnhandledHTTPRequestError(KeyError):
+    '''
+    Raised when a cassette does not c
+    ontain the request we want
+    '''
+    pass

vcr/stubs/__init__.py

@@ -4,10 +4,7 @@ from httplib import HTTPConnection, HTTPSConnection, HTTPMessage
 from cStringIO import StringIO
 
 from vcr.request import Request
-
-
-class CannotOverwriteExistingCassetteException(Exception):
-    pass
+from vcr.errors import CannotOverwriteExistingCassetteException
 
 
 def parse_headers(header_list):