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/WebDAV.hs

@@ -70,7 +70,8 @@ gen r u c gc = new <$> remoteCost gc expensiveRemoteCost
 			localpath = Nothing,
 			readonly = False,
 			availability = GloballyAvailable,
-			remotetype = remote
+			remotetype = remote,
+			mkUnavailable = gen r u (M.insert "url" "http://!dne!/" c) gc
 		}
 		chunkconfig = getChunkConfig c