mkdir should return ENOENT on Java 6 for non existent parent dirs

Corrected `mkdir` for Java 6 such that if there is a parent directory but it does not exist then `ENOENT` is thrown. Node actually isn't clear on what should happen here, but the `mkdirp` module [certainly expects `ENOENT`](https://github.com/substack/node-mkdirp/blob/master/index.js#L23). In addition the Trireme Java 7 version functions correctly with `mkdirp`. I would think that Java 7 works because a [FileNotFoundException](https://github.com/apigee/trireme/blob/master/core/src/main/java/io/apigee/trireme/core/modules/AsyncFilesystem.java#L218) is turned into `ENOENT` within AsyncFileSystem.

In addition all node exceptions within `mkdir` now communicate the path in the spirit of providing sufficient debug.