Add warning to note why LL cache data finding didn't work

This is so we get a better handle on why we fell back to gwdatafind.

gwcelery/tasks/detchar.py

@@ -85,6 +85,10 @@ def create_cache(ifo, start, end):
     if (cache_starttime <= start) and (end <= cache_endtime):
         # required data is in llhoft
         return cache
+    else:
+        log.warning('Requested data [%s, %s] not in low-latency cache '
+                    '[%s, %s]. Trying gwdatafind.',
+                    start, end, cache_starttime, cache_endtime)
 
     # otherwise, required data is not in the low latency cache
     high_latency = app.conf['high_latency_frame_types'][ifo]