java: Warn if classloader source does not exist
java: Warn if classloader source does not exist

Warn the user if a file or directory in classpath does not exist. This
makes it easier to spot cases where JAVA_HOME is not specified or is
specified incorrectly.

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@iki.fi>

penberg/hornet,penberg/hornet,penberg/hornet,penberg/hornet