Add a warning when an entry provided with "--classpath" does not exists

[jmini]

I lost a lot of time because I had a typo in the jar listed as --classpath value when running the command:

java -jar java-function-invoker-<version>.jar \
  --classpath build/my-function-1.0.0-SNAPSHOT-runner.jar \
  --target io.quarkus.gcp.functions.QuarkusHttpFunction

I think it would make sense to add a warning to help users (like me) realizing they added an invalid entry on the list provided with --classpath.