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/ReadPresentKey.hs

@@ -27,5 +27,5 @@ start (ks:us:[]) = do
 		then liftIO exitSuccess
 		else liftIO exitFailure
   where
-	k = fromMaybe (error "bad key") (file2key ks)
-start _ = error "Wrong number of parameters"
+	k = fromMaybe (giveup "bad key") (file2key ks)
+start _ = giveup "Wrong number of parameters"