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.

Assistant/Threads/Committer.hs

@@ -27,7 +27,6 @@ import qualified Utility.Lsof as Lsof
 import qualified Utility.DirWatcher as DirWatcher
 import Types.KeySource
 import Config
-import Annex.Exception
 import Annex.Content
 import Annex.Link
 import Annex.CatFile
@@ -217,7 +216,7 @@ commitStaged :: Annex Bool
 commitStaged = do
 	{- This could fail if there's another commit being made by
 	 - something else. -}
-	v <- tryAnnex Annex.Queue.flush
+	v <- tryNonAsync Annex.Queue.flush
 	case v of
 		Left _ -> return False
 		Right _ -> do