Add logging for the case where there's no response (but no exception) in install manager's check versions.

lib/tool_shed/galaxy_install/install_manager.py

@@ -1023,6 +1023,7 @@ def fetch_tool_versions( app, tool_shed_repository ):
         if text:
             return json.loads( text )
         else:
+            log.error("No content returned from tool shed repository version request to %s", url)
             failed_to_fetch = True
     except Exception:
         failed_to_fetch = True