Document "normal" recovery after panic file usage...

I was looking at https://github.com/github/gh-ost/pull/71 and wasn't sure if it made sense to have the error message that includes the normal recovery procedures. Or if in a panic, there is no normal recovery. Or if recovery is really so easy that basically it is just deleting the old file. If it is just a matter of, "In a normal recovery I would have deleted the _gst table and gone to have a beer." then this issue isn't needed. If there is more cleanup, then I think we should consider having something more than the, _peace out_ ✌️ ❤️ message. (I may be paraphrasing you infinity more accurate error message in the PR.)