Add slack notification on build failure #1438

Job	Run time
precommit / Setup Build Metadata	7s
precommit / pybf_build	13s
precommit / bf_build	37s
precommit / image_build	2m 56s
tests / allinone_image_test	2m 56s
tests / bf_image_test	2m 9s
tests / get_bf_versions	0s
tests / get_pybf_versions	0s
tests / pybf_cross_version_tests	0s
tests / bf_cross_version_tests	-1s
	8m 57s

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