improve error display when storing to an export/import remote fails

Prompted by the test suite on windows failing to with "export foo failed"
and no information about what went wrong.

Note that only storeExportWithContentIdentifier has been converted.
storeExport still returns a Bool and so exceptions may be hidden.

However, storeExportWithContentIdentifier has many more failure modes,
since it needs to avoid overwriting modified files. So it's more
important it have better error display.

Remote/Helper/ExportImport.hs

@@ -57,7 +57,7 @@ instance HasImportUnsupported (ImportActions Annex) where
 	importUnsupported = ImportActions
 		{ listImportableContents = return Nothing
 		, retrieveExportWithContentIdentifier = \_ _ _ _ _ -> return Nothing
-		, storeExportWithContentIdentifier = \_ _ _ _ _ -> return Nothing
+		, storeExportWithContentIdentifier = \_ _ _ _ _ -> return (Left "import not supported")
 		, removeExportWithContentIdentifier = \_ _ _ -> return False
 		, removeExportDirectoryWhenEmpty = Just $ \_ -> return False
 		, checkPresentExportWithContentIdentifier = \_ _ _ -> return False
@@ -154,8 +154,10 @@ adjustExportImport r = case M.lookup "exporttree" (config r) of
 					oldcids <- liftIO $ concat
 						<$> mapM (ContentIdentifier.getContentIdentifiers db (uuid r')) oldks
 					storeExportWithContentIdentifier (importActions r') f k loc oldcids p >>= \case
-						Nothing -> return False
-						Just newcid -> do
+						Left err -> do
+							warning err
+							return False
+						Right newcid -> do
 							withExclusiveLock gitAnnexContentIdentifierLock $ do
 								liftIO $ ContentIdentifier.recordContentIdentifier db (uuid r') newcid k
 								liftIO $ ContentIdentifier.flushDbQueue db