http.c: Reduce log spam

http.c:  Reduce log spam

Messages like "fwrite() failed: Connection reset by peer" are no
help whatsoever, especially since they can be caused simply by a
client disconnecting.

* Make those WARNINGs DEBUGs.
* Check the return of the headers fprintf.

Change-Id: I17bd5f3621514152a7b2b263c801324c5e96568b