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.

CmdLine/Action.hs

@@ -38,7 +38,7 @@ performCommandAction Command { cmdcheck = c, cmdname = name } seek cont = do
 	showerrcount =<< Annex.getState Annex.errcounter
   where
 	showerrcount 0 = noop
-	showerrcount cnt = error $ name ++ ": " ++ show cnt ++ " failed"
+	showerrcount cnt = giveup $ name ++ ": " ++ show cnt ++ " failed"
 
 {- Runs one of the actions needed to perform a command.
  - Individual actions can fail without stopping the whole command,