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.

Assistant/Upgrade.hs

@@ -153,7 +153,7 @@ upgradeToDistribution newdir cleanup distributionfile = do
   where
 	changeprogram program = liftIO $ do
 		unlessM (boolSystem program [Param "version"]) $
-			error "New git-annex program failed to run! Not using."
+			giveup "New git-annex program failed to run! Not using."
 		pf <- programFile
 		liftIO $ writeFile pf program