4.x: improve exception messages in stager plugin (#1081) #731

Job	Run time
build	5m 5s
shellcheck	3m 10s
checkstyle	3m 29s
copyright	3m 19s
javadoc	4m 5s
spotbugs	5m 23s
tests/cli-linux	4m 55s
cli/linux-amd64	5m 2s
tests/cli-functional-linux	6m 37s
cli/windows-amd64	5m 42s
tests/archetype-linux	4m 40s
vscode-ext	4m 6s
tests/linker-linux	4m 32s
cli/darwin-amd64	5m 11s
tests/others-linux	5m 40s
tests/cli-windows	6m 13s
tests/cli-functional-windows	8m 12s
tests/archetype-windows	5m 48s
tests/linker-windows	5m 34s
tests/others-windows	7m 22s
cli/binaries	9s
tests/results	3s
gate	1s
	1h 44m 18s

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