Add check for "unknown of type..." messages #2229

Job	Run time
maybe-expedite	2s
lint	3m 27s
test (third-party, bincode, stable)	38m 1s
test (third-party, bincode, nightly)	10m 53s
test (third-party, postcard, stable)	34m 26s
test (third-party, postcard, nightly)	10m 52s
test (other, bincode, stable)	24m 1s
test (other, bincode, nightly)	30m 6s
test (other, postcard, stable)	27m 19s
test (other, postcard, nightly)	32m 40s
all-checks	1s
	3h 31m 48s

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