4.x: improve exception messages in stager plugin #729

Job	Run time
copyright	3m 14s
build	4m 55s
shellcheck	3m 7s
checkstyle	3m 22s
spotbugs	5m 22s
javadoc	4m 9s
cli/linux-amd64	4m 53s
tests/cli-linux	5m 3s
cli/windows-amd64	5m 39s
vscode-ext	3m 53s
tests/cli-functional-linux	6m 30s
cli/darwin-amd64	5m 23s
tests/archetype-linux	4m 55s
tests/linker-linux	4m 44s
tests/others-linux	6m 18s
tests/cli-windows	5m 59s
tests/cli-functional-windows	7m 27s
tests/archetype-windows	5m 59s
tests/linker-windows	5m 20s
tests/others-windows	7m 39s
cli/binaries	5s
tests/results	4s
gate	1s
	1h 44m 1s

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