unify exception handling into Utility.Exception

Removed old extensible-exceptions, only needed for very old ghc.

Made webdav use Utility.Exception, to work after some changes in DAV's
exception handling.

Removed Annex.Exception. Mostly this was trivial, but note that
tryAnnex is replaced with tryNonAsync and catchAnnex replaced with
catchNonAsync. In theory that could be a behavior change, since the former
caught all exceptions, and the latter don't catch async exceptions.

However, in practice, nothing in the Annex monad uses async exceptions.
Grepping for throwTo and killThread only find stuff in the assistant,
which does not seem related.

Command.Add.undo is changed to accept a SomeException, and things
that use it for rollback now catch non-async exceptions, rather than
only IOExceptions.

Annex/Content.hs

@@ -56,7 +56,6 @@ import Annex.Perms
 import Annex.Link
 import Annex.Content.Direct
 import Annex.ReplaceFile
-import Annex.Exception
 
 #ifdef mingw32_HOST_OS
 import Utility.WinLock
@@ -167,7 +166,7 @@ lockContent key a = do
 	contentfile <- calcRepo $ gitAnnexLocation key
 	lockfile <- contentLockFile key
 	maybe noop setuplockfile lockfile
-	bracketAnnex (liftIO $ lock contentfile lockfile) (unlock lockfile) (const a)
+	bracket (liftIO $ lock contentfile lockfile) (unlock lockfile) (const a)
   where
 	alreadylocked = error "content is locked"
 	setuplockfile lockfile = modifyContent lockfile $
@@ -420,7 +419,7 @@ withObjectLoc key indirect direct = ifM isDirect
 cleanObjectLoc :: Key -> Annex () -> Annex ()
 cleanObjectLoc key cleaner = do
 	file <- calcRepo $ gitAnnexLocation key
-	void $ tryAnnexIO $ thawContentDir file
+	void $ tryIO $ thawContentDir file
 	cleaner
 	liftIO $ removeparents file (3 :: Int)
   where