Bug 4432: assertion failed: store.cc:1919: "isEmpty()"

Bug 4432: assertion failed: store.cc:1919: "isEmpty()"

When we failed to parse a response:

* Do not use parsing leftovers, such as HTTP response status code. Doing
  so screws up error detection logic in continueAfterParsingHeader() and
  leads to stuck transactions instead of error responses.

* Do not store the fake half-baked response (via replaceHttpReply).
  Doing so leads to assertions.  The fake response is only meant for
  continueAfterParsingHeader().

I also removed a misleading XXX about connection closure. Our
continueAfterParsingHeader() handles errors, not processReplyHeader().

TODO: The error detection/propagation code is ugly and should be
rewritten [using C++ exceptions].