Yield error if attempts_allowed is not int

For that exception we did get logged:

    ❯ grep Assertion /home/yoh/.local/state/dandi-cli/log/2024.12.16*
    /home/yoh/.local/state/dandi-cli/log/2024.12.16-15.24.12Z-119733.log:AssertionError: attempts_allowed_or_not is None of type <class 'NoneType'>

which makes little sense (besides some thread unsafe handling or None becoming
bool(True))). To overcome and avoid need for assert, just fold that
condition directly into "if" statement.

dandi/download.py

@@ -803,14 +803,9 @@ def _download_file(
                 attempts_allowed=attempts_allowed,
                 downloaded_in_attempt=downloaded_in_attempt,
             )
-            if not attempts_allowed:
+            if not isinstance(attempts_allowed_or_not, int) or not attempts_allowed:
                 yield {"status": "error", "message": str(exc)}
                 return
-            # for clear(er) typing, here we get only with int
-            assert isinstance(attempts_allowed_or_not, int), (
-                f"attempts_allowed_or_not is {attempts_allowed_or_not!r} "
-                f"of type {type(attempts_allowed_or_not)}"
-            )
             attempts_allowed = attempts_allowed_or_not
     else:
         lgr.warning("downloader logic: We should not be here!")