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.

Utility/DirWatcher/FSEvents.hs

@@ -17,7 +17,7 @@ import Data.Bits ((.&.))
 watchDir :: FilePath -> (FilePath -> Bool) -> Bool -> WatchHooks -> IO EventStream
 watchDir dir ignored scanevents hooks = do
 	unlessM fileLevelEventsSupported $
-		error "Need at least OSX 10.7.0 for file-level FSEvents"
+		giveup "Need at least OSX 10.7.0 for file-level FSEvents"
 	scan dir
 	eventStreamCreate [dir] 1.0 True True True dispatch
   where

Utility/DirWatcher/INotify.hs

@@ -152,7 +152,7 @@ watchDir i dir ignored scanevents hooks
 		-- disk full error.
 		| isFullError e =
 			case errHook hooks of
-				Nothing -> error $ "failed to add inotify watch on directory " ++ dir ++ " (" ++ show e ++ ")"
+				Nothing -> giveup $ "failed to add inotify watch on directory " ++ dir ++ " (" ++ show e ++ ")"
 				Just hook -> tooManyWatches hook dir
 		-- The directory could have been deleted.
 		| isDoesNotExistError e = return ()