`Jdk8Repackager` should not fail when optional libraries are missing

[Marcono1234]

It looks like Jdk8Repackager tries to process multiple library JARs, some of which are optional. When one of the libraries is missing, repackaging fails. For example the Temurin OpenJDK 8 builds (https://adoptium.net/releases.html?variant=openjdk8) do not seem to include jre/lib/javaws.jar.

Maybe Jdk8Repackager should only require that rt.jar exists and for all other libraries first check if they exist before trying to process them.

(Also it would be good to improve the error output here by joining the lines using \n.)