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/Schedule.hs

@@ -31,7 +31,7 @@ start = parse
 	parse (name:expr:[]) = go name $ \uuid -> do
 		showStart "schedile" name
 		performSet expr uuid
-	parse _ = error "Specify a repository."
+	parse _ = giveup "Specify a repository."
 
 	go name a = do
 		u <- Remote.nameToUUID name
@@ -47,7 +47,7 @@ performGet uuid = do
 
 performSet :: String -> UUID -> CommandPerform
 performSet expr uuid = case parseScheduledActivities expr of
-	Left e -> error $ "Parse error: " ++ e
+	Left e -> giveup $ "Parse error: " ++ e
 	Right l -> do
 		scheduleSet uuid l
 		next $ return True