enhance "hit paywall" error message to mention the probable cause

enhance "hit paywall" error message to mention the probable cause

Reporting errors from Done() is bad for progress reporting and such, so
factoring this out is a good idea and we start with moving the supposed-
to-be clearsigned file isn't clearsigned out first – improving the error
message in the process as we use the same message for a similar case
(NODATA) as this is what I have to look at with the venue wifi at
DebCamp and the old errormessage doesn't really say anything.