Avoid backtraces on expected failures when built with ghc 8; only use backtraces for unexpected errors.

ghc 8 added backtraces on uncaught errors. This is great, but git-annex was
using error in many places for a error message targeted at the user, in
some known problem case. A backtrace only confuses such a message, so omit it.

Notably, commands like git annex drop that failed due to eg, numcopies,
used to use error, so had a backtrace.

This commit was sponsored by Ethan Aubin.

Command/ContentLocation.hs

@@ -19,7 +19,7 @@ cmd = noCommit $ noMessages $
 
 run :: () -> String -> Annex Bool
 run _ p = do
-	let k = fromMaybe (error "bad key") $ file2key p
+	let k = fromMaybe (giveup "bad key") $ file2key p
 	maybe (return False) (\f -> liftIO (putStrLn f) >> return True)
 		=<< inAnnex' (pure True) Nothing check k
   where