rtsp: don't let CSeq error override earlier errors

[jay]

• When done, if an error has already occurred then don't check the
  sequence numbers for mismatch.

A sequence number may not have been received if an error occurred.

Prior to this change a sequence mismatch error would override earlier
errors. For example, a server that returns nothing would cause error
CURLE_GOT_NOTHING in Curl_http_done which was then overridden by
CURLE_RTSP_CSEQ_ERROR in rtsp_done.

Closes #xxxx

---

Before:

* multi_done: status: 0 prem: 0 done: 0
* Empty reply from server
* The CSseq of this request 1 did not match the response 0
* The cache now contains 0 members
* Closing connection 0
curl: (85) Empty reply from server

After:

* multi_done: status: 0 prem: 0 done: 0
* Empty reply from server
* The cache now contains 0 members
* Closing connection 0
curl: (52) Empty reply from server

[jay]

I can't think of anything going wrong with this but also I don't use the RTSP feature so one more call for comments before I land it.