Although regression window testing pin this to bug 1260276, I believe
this is a regression from bug 1126967. Bug 1260276 just make it more
visible because we stop automatically redirect users to the original
page.
This patch fix the bug by checking if the current page is in readerable
state (i.e. not error state), and send the message accordingly.
MozReview-Commit-ID: B5UJcPvVlAc
