More logging in HHWheelTimer::scheduleTimeoutFn

Summary:
This was supposed to be an update to D1720014 but I screwed up with arc.

Test Plan: run the test by hand and see the output

Reviewed By: jsedgwick@fb.com

Subscribers: exa, njormrod, folly-diffs@

FB internal diff: D1721951

Signature: t1:1721951:1417801009:72a27ec2ca473a996785ff9cae48bd51a43d9045

folly/io/async/HHWheelTimer.h

@@ -185,8 +185,11 @@ class HHWheelTimer : private folly::AsyncTimeout,
         try {
           fn_();
         } catch (std::exception const& e) {
-          LOG(ERROR) << e.what();
-        } catch (...) { }
+          LOG(ERROR) << "HHWheelTimer timeout callback threw an exception: "
+            << e.what();
+        } catch (...) {
+          LOG(ERROR) << "HHWheelTimer timeout callback threw a non-exception.";
+        }
         delete this;
       }
       F fn_;