In CRuby, an uncaught exception messages users with the exception's message, considering if it's overriden or not.

e.g.

class A < StandardError
  def message
    'hello'
  end
end

raise A

• CRuby stderr: /tmp/vu4TBd5/647:7:in `<main>': hello (A)
• JRuby stderr: A: A\n <main> at /tmp/vu4TBd5/648:7

This pull request changes JRuby behaviour similar to CRuby's.

• (patched) JRuby stderr: A: hello\n <main> at /tmp/vu4TBd5/650:7

Note that if the overriden message method raises an exception, it shouldn't message the internal exception, but the external exception, according to CRuby Bug #14566 https://bugs.ruby-lang.org/issues/14566

This pullreq already considers that.

e.g.

class A < StandardError
  def message
    raise 'boom boom!'
  end
end

raise A

• CRuby stderr: /tmp/vu4TBd5/655:7:in `<main>': A
• (patched) JRuby stderr: A: A\n <main> at /tmp/vu4TBd5/656:7"

I'm not fluent in Java, so please let me know if there's somewhere you don't feel good! I'm more than happy to update my commit.