java.cc: return on error rather than abort

Currently we call abort() if something is wrong whith
java.so command. Now that we allow to run processes from
the command line this may not be the best behavior because
a problem with starting a less import program will abort
the system:

  [/]% run java.so -cp java Hello
  run_elf(): running main() in the context of thread 0xffffc0000c30b010
  java.so: Can't create VM.
  Aborted

Instead of that we could just return and the system can
be still usable:

  [/]% run java.so -cp java Hello
  run_elf(): running main() in the context of thread 0xffffc0000c30b010
  java.so: Can't create VM.
  run: finished with exitcode 1

  [/]%

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
RunJava: handle directories in classpath properly

URLClassLoader determines whether URL denotes a jar or directory
by checking the last character of the path.

Before:

   [/]% java -cp java Hello
   Uncaught Java exception:
   java.lang.ClassNotFoundException: Hello
       at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
       at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
       at java.security.AccessController.doPrivileged(Native Method)
       at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
       at io.osv.RunJava.loadClass(RunJava.java:175)
       at io.osv.RunJava.runClass(RunJava.java:115)
       at io.osv.RunJava.parseArgs(RunJava.java:81)
       at io.osv.RunJava.main(RunJava.java:27)

After:

   [/]% java -cp java Hello
   Hello, world.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>