cache webmention endpoint discovery errors; drop cache expiration to 2h

before this, we only cached webmention endpoint discovery results when we successfully fetched the page. this is a bit aggressive, since it means we'll cache transient network failures, but with the lower expiration (2h instead of 1d), and now that the retry button flushes the cache (#524), i think it's ok.

for #534

tasks.py

@@ -510,22 +510,21 @@ def do_send_webmentions(self):
                      if 'DNS lookup failed for URL:' in str(e)
                      else {'code': 'EXCEPTION'})
 
+      if not cached:
+        memcache.set(cache_key, error if error else mention.receiver_endpoint,
+                     time=WEBMENTION_DISCOVERY_CACHE_TIME)
+
       if error is None:
         logging.info('Sent! %s', mention.response)
         self.record_source_webmention(mention)
         self.entity.sent.append(target)
-        if not cached:
-          memcache.set(cache_key, mention.receiver_endpoint,
-                       time=WEBMENTION_DISCOVERY_CACHE_TIME)
       else:
         code = error['code']
         status = error.get('http_status', 0)
         if (code == 'NO_ENDPOINT' or
             (code == 'BAD_TARGET_URL' and status == 204)):  # 204 is No Content
           logging.info('Giving up this target. %s', error)
           self.entity.skipped.append(target)
-          if not cached and code == 'NO_ENDPOINT':
-            memcache.set(cache_key, error, time=WEBMENTION_DISCOVERY_CACHE_TIME)
         elif code in ('BAD_TARGET_URL', 'RECEIVER_ERROR') and status / 100 == 4:
           # Give up on 4XX errors; we don't expect later retries to succeed.
           logging.info('Giving up this target. %s', error)