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/VCycle.hs

@@ -25,7 +25,7 @@ seek = withNothing start
 start ::CommandStart
 start = go =<< currentView
   where
-	go Nothing = error "Not in a view."
+	go Nothing = giveup "Not in a view."
 	go (Just v) = do
 		showStart "vcycle" ""
 		let v' = v { viewComponents = vcycle [] (viewComponents v) }