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/UserInfo.hs

@@ -16,6 +16,7 @@ module Utility.UserInfo (
 
 import Utility.Env
 import Utility.Data
+import Utility.Exception
 
 import System.PosixCompat
 import Control.Applicative
@@ -25,7 +26,7 @@ import Prelude
  -
  - getpwent will fail on LDAP or NIS, so use HOME if set. -}
 myHomeDir :: IO FilePath
-myHomeDir = either error return =<< myVal env homeDirectory
+myHomeDir = either giveup return =<< myVal env homeDirectory
   where
 #ifndef mingw32_HOST_OS
 	env = ["HOME"]