change retrieveExport and getKey to throw exception

retrieveExport is part of ongoing transition to make remote methods
throw exceptions, rather than silently hide them.

getKey very rarely fails, and when it does it's always for the same reason
(user configured annex.backend to url for some reason). So, this will
avoid dealing with Nothing everywhere it's used.

This commit was sponsored by Ilya Shlyakhter on Patreon.

Types/Remote.hs

@@ -236,7 +236,8 @@ data ExportActions a = ExportActions
 	-- Retrieves exported content to a file.
 	-- (The MeterUpdate does not need to be used if it writes
 	-- sequentially to the file.)
-	, retrieveExport :: Key -> ExportLocation -> FilePath -> MeterUpdate -> a Bool
+	-- Throws exception on failure.
+	, retrieveExport :: Key -> ExportLocation -> FilePath -> MeterUpdate -> a ()
 	-- Removes an exported file (succeeds if the contents are not present)
 	, removeExport :: Key -> ExportLocation -> a Bool
 	-- Removes an exported directory. Typically the directory will be
@@ -269,15 +270,17 @@ data ImportActions a = ImportActions
 	-- This has to be used rather than retrieveExport
 	-- when a special remote supports imports, since files on such a
 	-- special remote can be changed at any time.
+	--
+	-- Throws exception on failure.
 	, retrieveExportWithContentIdentifier 
 		:: ExportLocation
 		-> ContentIdentifier
 		-- file to write content to
 		-> FilePath
 		-- callback that generates a key from the downloaded content
-		-> a (Maybe Key)
+		-> a Key
 		-> MeterUpdate
-		-> a (Maybe Key)
+		-> a Key
 	-- Exports content to an ExportLocation, and returns the
 	-- ContentIdentifier corresponding to the content it stored.
 	--