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.

Command/Map.hs

@@ -200,7 +200,7 @@ tryScan r
 	  where
 		p = proc cmd $ toCommand params
 
-	configlist = Ssh.onRemote r (pipedconfig, Nothing) "configlist" [] []
+	configlist = Ssh.onRemote r (pipedconfig, return Nothing) "configlist" [] []
 	manualconfiglist = do
 		gc <- Annex.getRemoteGitConfig r
 		sshparams <- Ssh.toRepo r gc [Param sshcmd]