Ironically, when jpackage is executed with the --verbose option, the final error messages are not printed to stderr:

$ ./build/windows-x64/jdk/bin/jpackage --type app-image --input app --main-jar hello.jar --name Hello 1> /dev/null
Bundler Windows Application Image skipped because of a configuration problem: A main class was not specified nor was one found in the jar hello.jar
Advice to fix: Specify a main class or ensure that the jar hello.jar specifies one in the manifest

$ ./build/windows-x64/jdk/bin/jpackage --type app-image --input app --main-jar hello.jar --name Hello --verbose 1> /dev/null

In the above commands, stdout is redirected to /dev/null. The only output is stderr. It contains error messages if jpackage is executed without the --verbose option and is empty otherwise.