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.

Command/Reinject.hs

@@ -55,15 +55,13 @@ startSrcDest _ = giveup "specify a src file and a dest file"
 startKnown :: FilePath -> CommandStart
 startKnown src = notAnnexed src $
 	starting "reinject" (ActionItemOther (Just src)) $ do
-		mkb <- genKey ks nullMeterUpdate Nothing
-		case mkb of
-			Nothing -> error "Failed to generate key"
-			Just (key, _) -> ifM (isKnownKey key)
-				( perform src key
-				, do
-					warning "Not known content; skipping"
-					next $ return True
-				)
+		(key, _) <- genKey ks nullMeterUpdate Nothing
+		ifM (isKnownKey key)
+			( perform src key
+			, do
+				warning "Not known content; skipping"
+				next $ return True
+			)
   where
 	ks = KeySource src' src' Nothing
 	src' = toRawFilePath src