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/AddUnused.hs

@@ -38,4 +38,4 @@ perform key = next $ do
  - it seems better to error out, rather than moving bad/tmp content into
  - the annex. -}
 performOther :: String -> Key -> CommandPerform
-performOther other _ = error $ "cannot addunused " ++ other ++ "content"
+performOther other _ = giveup $ "cannot addunused " ++ other ++ "content"