Make the VSL error string on HTTP fetch timeout errors be 'http first

read error' when tripping on first_byte_timeout, and 'http read error'
when tripping on between_bytes_timeout (used to be 'http first read
error' in both cases). This now matches current master.

bin/varnishd/cache_fetch.c

@@ -498,7 +498,7 @@ FetchHdr(struct sess *sp)
 		i = HTC_Rx(w->htc);
 		if (i < 0) {
 			WSP(sp, SLT_FetchError,
-			    "http first read error: %d %d (%s)",
+			    "http read error: %d %d (%s)",
 			    i, errno, strerror(errno));
 			VDI_CloseFd(sp);
 			/* XXX: other cleanup ? */