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/AutoCorrect.hs

@@ -50,7 +50,7 @@ prepare input showmatch matches r =
 			| otherwise -> sleep n
 		Nothing -> list
   where
-	list = error $ unlines $
+	list = giveup $ unlines $
 		[ "Unknown command '" ++ input ++ "'"
 		, ""
 		, "Did you mean one of these?"