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/ExamineKey.hs

@@ -21,6 +21,6 @@ cmd = noCommit $ noMessages $ dontCheck repoExists $
 
 run :: Maybe Utility.Format.Format -> String -> Annex Bool
 run format p = do
-	let k = fromMaybe (error "bad key") $ file2key p
+	let k = fromMaybe (giveup "bad key") $ file2key p
 	showFormatted format (key2file k) (keyVars k)
 	return True