For example if I run following commands on Linux:
jpackage create-installer aaa --input input --output output --name Test --main-jar hello.jar --class Hello --force --files hello.jar
create-installer dmg --input input --output output --name Test --main-jar hello.jar --class Hello --force --files hello.jar

jpackage will exit silently without displaying any errors messages. Exit code will be set to 255 which seems correct, since no bundles are produced. With --verbose it also does not show any error messages.

We should display something "Invalid installer type 'aaa' or maybe 'dmg' is not supported on this platform.