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/Undo.hs

@@ -32,7 +32,7 @@ seek ps = do
 	-- in the index.
 	(fs, cleanup) <- inRepo $ LsFiles.notInRepo False ps
 	unless (null fs) $
-		error $ "Cannot undo changes to files that are not checked into git: " ++ unwords fs
+		giveup $ "Cannot undo changes to files that are not checked into git: " ++ unwords fs
 	void $ liftIO $ cleanup
 
 	-- Committing staged changes before undo allows later