improve http response when err happens.

[suzp1984]

1. response 200 with code when rtc/v1 remote sdp parse error;
2. response 500 when no handled err.

Try to improve two problem:

1. let rtc/v1/[whep|whip] response 200 with body {code : 400} when remote_sdp parse error happen by follow rtc/v1/[play | publish] api's response. The current behavior in this scenario is close the connection without response anything.

2. Let SrsHttpServeMux response 500 when err not handled, the current behavior is also to close the socket connection without response anything.