Fix fallback to default partials on templates.

If the directory containing a template does not contain
the partial, it should be sought in the default data files.
Closes #7164.

src/Text/Pandoc/Templates.hs

@@ -34,6 +34,7 @@ import Control.Monad.Except (catchError, throwError)
 import Data.Text (Text)
 import qualified Data.Text as T
 import Text.Pandoc.Error
+import System.IO.Error (isDoesNotExistError)
 
 -- | Wrap a Monad in this if you want partials to
 -- be taken only from the default data files.
@@ -70,6 +71,9 @@ getTemplate tp = UTF8.toText <$>
              PandocResourceNotFound _ ->
                 -- see #5987 on reason for takeFileName
                 readDataFile ("templates" </> takeFileName tp)
+             PandocIOError _ ioe | isDoesNotExistError ioe ->
+                -- see #5987 on reason for takeFileName
+                readDataFile ("templates" </> takeFileName tp)
              _ -> throwError e))
 
 -- | Get default template for the specified writer.