diff --git a/oauth2client/client.py b/oauth2client/client.py
index 3d7ded7..c8d0941 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -36,7 +36,12 @@
   pass
 
 
-class RequestError(Error):
+class FlowExchangeError(Error):
+  """Error occurred during request."""
+  pass
+
+
+class AccessTokenRefreshError(Error):
   """Error occurred during request."""
   pass
 
@@ -144,7 +149,7 @@
   @property
   def invalid(self):
     """True if the credentials are invalid, such as being revoked."""
-    return self._invalid
+    return getattr(self, '_invalid', False)
 
   def set_store(self, store):
     """Set the storage for the credential.
@@ -204,16 +209,18 @@
     else:
       # An {'error':...} response body means the token is expired or revoked, so
       # we flag the credentials as such.
+      logging.error('Failed to retrieve access token: %s' % content)
+      error_msg = 'Invalid response %s.' % resp['status']
       try:
         d = simplejson.loads(content)
         if 'error' in d:
+          error_msg = d['error']
           self._invalid = True
           if self.store is not None:
             self.store(self)
       except:
         pass
-      logging.error('Failed to retrieve access token: %s' % content)
-      raise RequestError('Invalid response %s.' % resp['status'])
+      raise AccessTokenRefreshError(error_msg)
 
   def authorize(self, http):
     """Authorize an httplib2.Http instance with these credentials.
@@ -258,6 +265,7 @@
       if resp.status == 401:
         logging.info("Refreshing because we got a 401")
         self._refresh(request_orig)
+        headers['authorization'] = 'OAuth ' + self.access_token
         return request_orig(uri, method, body, headers,
                             redirections, connection_type)
       else:
@@ -378,13 +386,14 @@
     parts[4] = urllib.urlencode(query)
     return urlparse.urlunparse(parts)
 
-  def step2_exchange(self, code):
+  def step2_exchange(self, code, http=None):
     """Exhanges a code for OAuth2Credentials.
 
     Args:
       code: string or dict, either the code as a string, or a dictionary
         of the query parameters to the redirect_uri, which contains
         the code.
+      http: httplib2.Http, optional http instance to use to do the fetch
     """
 
     if not (isinstance(code, str) or isinstance(code, unicode)):
@@ -402,8 +411,9 @@
       'user-agent': self.user_agent,
       'content-type': 'application/x-www-form-urlencoded'
     }
-    h = httplib2.Http()
-    resp, content = h.request(self.token_uri, method='POST', body=body, headers=headers)
+    if http is None:
+      http = httplib2.Http()
+    resp, content = http.request(self.token_uri, method='POST', body=body, headers=headers)
     if resp.status == 200:
       # TODO(jcgregorio) Raise an error if simplejson.loads fails?
       d = simplejson.loads(content)
@@ -419,4 +429,12 @@
                                self.user_agent)
     else:
       logging.error('Failed to retrieve access token: %s' % content)
-      raise RequestError('Invalid response %s.' % resp['status'])
+      error_msg = 'Invalid response %s.' % resp['status']
+      try:
+        d = simplejson.loads(content)
+        if 'error' in d:
+          error_msg = d['error']
+      except:
+        pass
+
+      raise FlowExchangeError(error_msg)