fix(downloads): a download larger than the space left says so, not "processing failed"

yt-dlp refuses a file announced over `max_filesize` by logging one line and returning with no file
(measured, and pinned in test_formats against a local server): the worker found no output and
failed the job as `postprocess`. `formats.watching_size_cap` now sees that line through our yt-dlp
logger, and the download (and, should it ever happen, a YouTube clip's range fetch) fails as the
owner's refusal with DOWNLOAD_TOO_BIG_MESSAGE, classified `quota_exceeded`. The download's quota
copy says it does not fit in what is left — true at a full cap too — instead of "Storage full".
A pre-existing type error the change touched is fixed (baseline down by one).

backend/tests/test_download_errors.py

@@ -65,6 +65,7 @@ def test_our_own_worker_refusals_classify_as_themselves():
     assert classify(dlerrors.QUOTA_FULL_MESSAGE) == "quota_exceeded"
     assert classify(dlerrors.CLIP_TOO_BIG_MESSAGE) == "quota_exceeded"
     assert classify(dlerrors.CONVERTED_TOO_BIG_MESSAGE) == "quota_exceeded"
+    assert classify(dlerrors.DOWNLOAD_TOO_BIG_MESSAGE) == "quota_exceeded"
 
 
 def test_the_redacted_network_text_classifies_back_to_network():