Non existing external modules should be ignored in ManifestClassPathProcessor

If MANIFEST.MF Class-Path points to invalid external path,
it should be ignored, and warning logged.

Instead of failing whole deployment,
where actually it could still work.

e.g.

18:01:45,857 ERROR [org.jboss.msc.service.fail] (MSC service thread 1-5) MSC00001: Failed to start service jboss.module.spec.service."deployment.external./home/codegen/dependencies/java/stable/dependencies/libs/guava-11.0.1.jar".main: org.jboss.msc.service.StartException in service jboss.module.spec.service."deployment.external./home/codegen/dependencies/java/stable/dependencies/libs/guava-11.0.1.jar".main: java.io.FileNotFoundException: /home/codegen/dependencies/java/stable/dependencies/libs/guava-11.0.1.jar (No such file or directory)
at org.jboss.as.server.moduleservice.ExternalModuleSpecService.start(ExternalModuleSpecService.java:65) [jboss-as-server-7.2.0.Alpha1-SNAPSHOT.jar:7.2.0.Alpha1-SNAPSHOT]