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/DiffDriver.hs

@@ -73,7 +73,7 @@ parseReq opts = case separate (== "--") opts of
 	mk (unmergedpath:[]) = UnmergedReq { rPath = unmergedpath }
 	mk _ = badopts
 
-	badopts = error $ "Unexpected input: " ++ unwords opts
+	badopts = giveup $ "Unexpected input: " ++ unwords opts
 
 {- Check if either file is a symlink to a git-annex object,
  - which git-diff will leave as a normal file containing the link text.