MINOR: hlua: Add error message relative to the Channel manipulation and HTTP mode

When the developper try to manipulate HAProxy channels in HTTP mode,
an error throws without explanation. This patch adds an explanation.
diff --git a/src/hlua.c b/src/hlua.c
index 6ec69e4..1eac9d1 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -2852,8 +2852,10 @@
 
 	chn = MAY_LJMP(hlua_checkchannel(L, 1));
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	if (_hlua_channel_dup(chn, L) == 0)
 		MAY_LJMP(hlua_yieldk(L, 0, 0, hlua_channel_dup_yield, TICK_ETERNITY, 0));
@@ -2880,8 +2882,10 @@
 
 	chn = MAY_LJMP(hlua_checkchannel(L, 1));
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	ret = _hlua_channel_dup(chn, L);
 	if (unlikely(ret == 0))
@@ -2920,8 +2924,10 @@
 
 	chn = MAY_LJMP(hlua_checkchannel(L, 1));
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	ret = ci_getline_nc(chn, &blk1, &len1, &blk2, &len2);
 	if (ret == 0)
@@ -2968,8 +2974,10 @@
 	int ret;
 	int max;
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	/* Check if the buffer is available because HAProxy doesn't allocate
 	 * the request buffer if its not required.
@@ -3040,8 +3048,10 @@
 	chn = MAY_LJMP(hlua_checkchannel(L, 1));
 	lua_pushinteger(L, 0);
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	b_set_data(&chn->buf, co_data(chn));
 
@@ -3062,8 +3072,10 @@
 	int max;
 	struct hlua *hlua = hlua_gethlua(L);
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	if (unlikely(channel_output_closed(chn))) {
 		lua_pushinteger(L, -1);
@@ -3166,8 +3178,10 @@
 
 	chn = MAY_LJMP(hlua_checkchannel(L, 1));
 
-	if (chn_strm(chn)->be->mode == PR_MODE_HTTP)
+	if (chn_strm(chn)->be->mode == PR_MODE_HTTP) {
+		lua_pushfstring(L, "Cannot manipulate HAProxy channels in HTTP mode.");
 		WILL_LJMP(lua_error(L));
+	}
 
 	len = MAY_LJMP(luaL_checkinteger(L, 2));
 	l = MAY_LJMP(luaL_checkinteger(L, -1));