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.

Git/CurrentRepo.hs

@@ -52,7 +52,7 @@ get = do
 		curr <- getCurrentDirectory
 		Git.Config.read $ newFrom $
 			Local { gitdir = absd, worktree = Just curr }
-	configure Nothing Nothing = error "Not in a git repository."
+	configure Nothing Nothing = giveup "Not in a git repository."
 
 	addworktree w r = changelocation r $
 		Local { gitdir = gitdir (location r), worktree = w }