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.

Git/UpdateIndex.hs

@@ -29,8 +29,6 @@ import Git.Command
 import Git.FilePath
 import Git.Sha
 
-import Control.Exception (bracket)
-
 {- Streamers are passed a callback and should feed it lines in the form
  - read by update-index, and generated by ls-tree. -}
 type Streamer = (String -> IO ()) -> IO ()