I would like that the installer would just warn the user instead aborting the installation.
If you do not want the autodetection action to exit after throwing an error, you can just add:
<abortOnError>0</abortOnError>
inside the <autodetectJava> action tag. If you want to throw your own custom message, you can also add a <customErrorMessage> tag. Alternatively, you can add a <showMessageOnError>0</showMessageOnError> so it will fail silently and then, later in the installer you can show a specific installation page or issue a custom <showWarning> action based on the presence of the environment java_version.