4.x: improve exception messages in stager plugin #730

Job	Run time
copyright	3m 22s
checkstyle	3m 27s
build	5m 0s
shellcheck	3m 8s
cli/linux-amd64	4m 36s
spotbugs	5m 19s
tests/cli-linux	4m 58s
cli/windows-amd64	6m 13s
javadoc	3m 58s
tests/cli-functional-linux	6m 26s
cli/darwin-amd64	5m 16s
vscode-ext	4m 1s
tests/archetype-linux	4m 55s
tests/linker-linux	4m 37s
tests/others-linux	5m 50s
tests/cli-windows	6m 39s
tests/cli-functional-windows	7m 16s
tests/archetype-windows	5m 40s
tests/linker-windows	5m 23s
tests/others-windows	7m 19s
cli/binaries	8s
tests/results	3s
gate	3s
	1h 43m 37s

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