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/Glob.hs

@@ -12,6 +12,8 @@ module Utility.Glob (
 	matchGlob
 ) where
 
+import Utility.Exception
+
 import System.Path.WildMatch
 
 import "regex-tdfa" Text.Regex.TDFA
@@ -26,7 +28,7 @@ compileGlob :: String -> GlobCase -> Glob
 compileGlob glob globcase = Glob $
 	case compile (defaultCompOpt {caseSensitive = casesentitive}) defaultExecOpt regex of
 		Right r -> r
-		Left _ -> error $ "failed to compile regex: " ++ regex
+		Left _ -> giveup $ "failed to compile regex: " ++ regex
   where
 	regex = '^':wildToRegex glob
 	casesentitive = case globcase of