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/LockContent.hs

@@ -32,7 +32,7 @@ start [ks] = do
 		then exitSuccess
 		else exitFailure
   where
-	k = fromMaybe (error "bad key") (file2key ks)
+	k = fromMaybe (giveup "bad key") (file2key ks)
 	locksuccess = ifM (inAnnex k)
 		( liftIO $ do
 			putStrLn contentLockedMarker
@@ -41,4 +41,4 @@ start [ks] = do
 			return True
 		, return False
 		)
-start _ = error "Specify exactly 1 key."
+start _ = giveup "Specify exactly 1 key."