Fix accidental page watching when setting missing

Fix issue reported on the RedWarn talk page (point 2). For some reason, it seems the rwWatchTime property of rw.config would be undefined if the setting had not yet been changed - perhaps this is indicative of a larger issue with config? This patch will just check that the property exists on rw.config before watching the page.

Sorry that I haven't opened an issue here for this - if you'd like me to, I'll happily oblige.