Post the test output log separately from the test result, fixes bug when non-UTF8 chars are in logs.
Be more defensive when posting the result.

* Catch all classes of errors in rt:post_result/2.
* Avoid running mochijson2:encode/1 inside the catch block, return
  `error` from the block.
* Match on the return value of rt:post_result/2 in
  giddyup:post_result/1.
Merge pull request #500 from basho/bugfix/sdc/log-utf8-post

Post the test output log separately from the test result