testremote: Add testing of behavior when remote is not available

Added a mkUnavailable method, which a Remote can use to generate a version
of itself that is not available. Implemented for several, but not yet all
remotes.

This allows testing that checkPresent properly throws an exceptions when
it cannot check if a key is present or not. It also allows testing that the
other methods don't throw exceptions in these circumstances.

This immediately found several bugs, which this commit also fixes!

* git remotes using ssh accidentially had checkPresent return
  an exception, rather than throwing it
* The chunking code accidentially returned False rather than
  propigating an exception when there were no chunks and
  checkPresent threw an exception for the non-chunked key.

This commit was sponsored by Carlo Matteo Capocasa.

Remote/External.hs

@@ -65,7 +65,9 @@ gen r u c gc = do
 			gitconfig = gc,
 			readonly = False,
 			availability = avail,
-			remotetype = remote
+			remotetype = remote,
+			mkUnavailable = gen r u c $
+				gc { remoteAnnexExternalType = Just "!dne!" }
 		}
   where
 	externaltype = fromMaybe (error "missing externaltype") (remoteAnnexExternalType gc)