Various fixes and features

* renamed the confusing method name "capture_core" to "send_report".
* fixed the comment for function "catcher" for it no longer sends the
  report.
* fixed the err callback function in method "call" to return the "err"
  argument.
* added new method "gen_capture_err" to generate a "err" callback
  function that can be fed into xpcall(), so as to prevent dynamic closure
  creations in the user's code. also added a test case for it.
* http_send_core(): fixed the error handling of bad HTTP response status
  line: the status variable can take the nil value.