upstream_rebuilds.py: catch AttributeError #500

Job	Run time
unittests	9s
sanity	23s
jammy, good, verbose=true / build debs	5m 27s
focal, broken, cont-on-err / build debs	4m 13s
bookworm, good, verbose=bloom / build debs	8m 1s
focal, broken / build debs	2m 43s
jammy, good, timeout=3 / build debs	3m 2s
jammy push / deploy	0s
bookworm squash / deploy	0s
	23m 58s

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