Improve diagnostics when an invalid triplet name is supplied. #1604

Job	Run time
json-schema	59s
format	3m 23s
builds / build (windows-2022, windows-ci)	5m 5s
builds / build (ubuntu-20.04, linux-ci)	3m 38s
builds / build (macos-12, macos-ci)	3m 20s
	16m 25s

Provide feedback