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]

Remote/Helper/Encryptable.hs

@@ -66,14 +66,14 @@ encryptionSetup c gc = do
 			encsetup $ genEncryptedCipher cmd (c, gc) key Hybrid
 		Just "pubkey" -> encsetup $ genEncryptedCipher cmd (c, gc) key PubKey
 		Just "sharedpubkey" -> encsetup $ genSharedPubKeyCipher cmd key
-		_ -> error $ "Specify " ++ intercalate " or "
+		_ -> giveup $ "Specify " ++ intercalate " or "
 			(map ("encryption=" ++)
 				["none","shared","hybrid","pubkey", "sharedpubkey"])
 			++ "."
-	key = fromMaybe (error "Specifiy keyid=...") $ M.lookup "keyid" c
+	key = fromMaybe (giveup "Specifiy keyid=...") $ M.lookup "keyid" c
 	newkeys = maybe [] (\k -> [(True,k)]) (M.lookup "keyid+" c) ++
 		maybe [] (\k -> [(False,k)]) (M.lookup "keyid-" c)
-	cannotchange = error "Cannot set encryption type of existing remotes."
+	cannotchange = giveup "Cannot set encryption type of existing remotes."
 	-- Update an existing cipher if possible.
 	updateCipher cmd v = case v of
 		SharedCipher _ | maybe True (== "shared") encryption -> return (c', EncryptionIsSetup)

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

Remote/Helper/Messages.hs

@@ -29,7 +29,7 @@ showChecking :: Describable a => a -> Annex ()
 showChecking v = showAction $ "checking " ++ describe v
 
 cantCheck :: Describable a => a -> e
-cantCheck v = error $ "unable to check " ++ describe v
+cantCheck v = giveup $ "unable to check " ++ describe v
 
 showLocking :: Describable a => a -> Annex ()
 showLocking v = showAction $ "locking " ++ describe v

Remote/Helper/Ssh.hs

@@ -29,7 +29,7 @@ import Config
 toRepo :: Git.Repo -> RemoteGitConfig -> [CommandParam] -> Annex [CommandParam]
 toRepo r gc sshcmd = do
 	let opts = map Param $ remoteAnnexSshOptions gc
-	let host = fromMaybe (error "bad ssh url") $ Git.Url.hostuser r
+	let host = fromMaybe (giveup "bad ssh url") $ Git.Url.hostuser r
 	params <- sshOptions (host, Git.Url.port r) gc opts
 	return $ params ++ Param host : sshcmd