Error.stack needs trimming

Error.stack in other JavaScript engines is usually a string, and has different contents. To help embedders switching from other JavaScript engines, nashorn should present Error.stack in a compatible way. Consider the following -

function error() {
  try {
    throw new Error('foo');
  } catch (e) {
    var s = e.stack;
    print(typeof s + ' ' + Array.isArray(s));
    print(s);
  }
}
error();

when run with v8, this prints -

string false
Error: foo
    at error (error.js:3:11)
    at error.js:11:1

when run with spidermonkey, this prints -

string false
error@error.js:3
@error.js:11

when run with nashorn, this prints -

object true
jdk.nashorn.internal.scripts.Script$error.error(error.js:3),jdk.nashorn.internal.scripts.Script$error.runScript(error.js:11)

The 'jdk.nashorn.internal.scripts.Script$error' is unnecessary and should be removed. runScript is also internal and should be removed. The array should be converted to a String.