Show a user-facing message instead of a traceback on invalid file type

While our importing dialog limits the types of files that can be selected,
users can also import by providing a filename on the command line.

ftl/core/importing.ftl

@@ -87,3 +87,4 @@ importing-processed-notes =
         [one] Processed { $count } note...
        *[other] Processed { $count } notes...
     }
+importing-unable-to-import-filename = Unable to import { $filename }: file type not supported

qt/aqt/import_export/importing.py

@@ -25,7 +25,11 @@ def import_file(mw: aqt.main.AnkiQt, path: str) -> None:
     elif filename.endswith(".apkg"):
         import_anki_package(mw, path)
     else:
-        raise NotImplementedError
+        showWarning(
+            tr.importing_unable_to_import_filename(filename=filename),
+            parent=mw,
+            textFormat="plain",
+        )
 
 
 def prompt_for_file_then_import(mw: aqt.main.AnkiQt) -> None: