Fixes for dx get applet-xxxx

* Fix retrieval of properties, which was broken before
* Retrieve details
* Do not change applet semantics (in particular, inputSpec/outputSpec)
  when doing cleanup
Java bindings: simplify checkNotNull assertions.

* Provide better assertion messages for all checkNotNull assertions.
* Fix a bug in these assertions in InputParameter and OutputParameter.
Use new isDeveloperFor check to determine whether app is writable
Accept all HTTP status codes in the 200 series as OK
Minor cleanup.
Ignore InvalidState when uploading an empty part for file closing