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.

Types/Remote.hs

@@ -95,7 +95,10 @@ data RemoteA a = Remote {
 	-- a Remote can be globally available. (Ie, "in the cloud".)
 	availability :: Availability,
 	-- the type of the remote
-	remotetype :: RemoteTypeA a
+	remotetype :: RemoteTypeA a,
+	-- For testing, makes a version of this remote that is not
+	-- available for use. All its actions should fail.
+	mkUnavailable :: a (Maybe (RemoteA a))
 }
 
 instance Show (RemoteA a) where