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.hs

@@ -64,14 +64,16 @@ import Utility.Quvi (QuviVersion)
 import Utility.InodeCache
 
 import "mtl" Control.Monad.Reader
-import Control.Monad.Catch
 import Control.Concurrent
 import qualified Data.Map as M
 import qualified Data.Set as S
 
 {- git-annex's monad is a ReaderT around an AnnexState stored in a MVar.
- - This allows modifying the state in an exception-safe fashion.
  - The MVar is not exposed outside this module.
+ -
+ - Note that when an Annex action fails and the exception is caught,
+ - ny changes the action has made to the AnnexState are retained,
+ - due to the use of the MVar to store the state.
  -}
 newtype Annex a = Annex { runAnnex :: ReaderT (MVar AnnexState) IO a }
 	deriving (