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.

Remote/Helper/Http.hs

@@ -70,7 +70,7 @@ handlePopper numchunks chunksize meterupdate h sink = do
 -- meter as it goes.
 httpBodyRetriever :: FilePath -> MeterUpdate -> Response BodyReader -> IO ()
 httpBodyRetriever dest meterupdate resp
-	| responseStatus resp /= ok200 = error $ show $ responseStatus resp
+	| responseStatus resp /= ok200 = giveup $ show $ responseStatus resp
 	| otherwise = bracket (openBinaryFile dest WriteMode) hClose (go zeroBytesProcessed)
   where
 	reader = responseBody resp