proper error reporting of post requests incl. to yarrg

proper error reporting of post requests incl. to yarrg

ClientHttpRequest.java:
* .post() returns a boolean saying whether the submission was
  successful; you have to call .resultstream() to get the stream.

MarketUploader.java:
* Factor out StringBuild and repeated readLine into new function
  readstreamstring.
* Report http errors occurring in sendInitialData (part of PCTB
  upload) and have it return "null" in that case.
* New helper function post_for_yarrg that does the post and puts up a
  dialogue box if it's not right.  Use it in the two relevant places.