pagecache: Show memcached errors in app logs when writes fail.

This gives us a little more information on failed writes to the
pagecache, though we probably won't see any because of no_reply mode.

r2/r2/controllers/reddit_base.py

@@ -674,9 +674,11 @@ class MinimalController(BaseController):
                 g.rendercache.set(self.request_key(),
                                   (response, c.cookies),
                                   g.page_cache_time)
-            except MemcachedError:
-                # the key was too big to set in the rendercache
-                g.log.debug("Ignored too-big render cache")
+            except MemcachedError as e:
+                # this codepath will actually never be hit as long as
+                # the pagecache memcached client is in no_reply mode.
+                g.log.warning("Ignored exception (%r) on pagecache "
+                              "write for %r", e, request.path)
 
         # send cookies
         for k,v in c.cookies.iteritems():