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/Web.hs

@@ -61,7 +61,8 @@ gen r _ c gc =
 		repo = r,
 		readonly = True,
 		availability = GloballyAvailable,
-		remotetype = remote
+		remotetype = remote,
+		mkUnavailable = return Nothing
 	}
 
 downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool