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/Chunked.hs

@@ -59,7 +59,7 @@ getChunkConfig m =
 		Just size
 			| size == 0 -> NoChunks
 			| size > 0 -> c (fromInteger size)
-		_ -> error $ "bad configuration " ++ f ++ "=" ++ v
+		_ -> giveup $ "bad configuration " ++ f ++ "=" ++ v
 
 -- An infinite stream of chunk keys, starting from chunk 1.
 newtype ChunkKeyStream = ChunkKeyStream [Key]