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.

Utility/Exception.hs

@@ -1,6 +1,6 @@
 {- Simple IO exception handling (and some more)
  -
- - Copyright 2011-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2016 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -10,6 +10,7 @@
 
 module Utility.Exception (
 	module X,
+	giveup,
 	catchBoolIO,
 	catchMaybeIO,
 	catchDefaultIO,
@@ -40,6 +41,17 @@ import GHC.IO.Exception (IOErrorType(..))
 
 import Utility.Data
 
+{- Like error, this throws an exception. Unlike error, if this exception
+ - is not caught, it won't generate a backtrace. So use this for situations
+ - where there's a problem that the user is excpected to see in some
+ - circumstances. -}
+giveup :: [Char] -> a
+#if MIN_VERSION_base(4,9,0)
+giveup = errorWithoutStackTrace
+#else
+giveup = error
+#endif
+
 {- Catches IO errors and returns a Bool -}
 catchBoolIO :: MonadCatch m => m Bool -> m Bool
 catchBoolIO = catchDefaultIO False