Throw an error when a chatnet has no available url

If you type /connect <CN> and the chatnet <CN> has no url available
let's just throw an error instead of trying to process <CN> as a url.

src/fe-common/core/fe-core-commands.c

@@ -51,7 +51,8 @@ static int ret_texts[] = {
         TXT_INVALID_TIME,
         TXT_INVALID_CHARSET,
         TXT_EVAL_MAX_RECURSE,
-        TXT_PROGRAM_NOT_FOUND
+        TXT_PROGRAM_NOT_FOUND,
+	TXT_NO_SERVER_DEFINED,
 };
 
 int command_hide_output;

src/fe-common/core/module-formats.c

@@ -223,6 +223,7 @@ FORMAT_REC fecommon_core_formats[] = {
 	{ "invalid_charset", "Invalid charset: $0", 1, { 0 } },
 	{ "eval_max_recurse", "/eval hit maximum recursion limit", 0 },
 	{ "program_not_found", "Could not find file or file is not executable", 0 },
+	{ "no_server_defined", "No servers defined for this network, see /help server for how to add one", 0 },
 
 	/* ---- */
 	{ NULL, "Themes", 0 },

src/fe-common/core/module-formats.h

@@ -192,6 +192,7 @@ enum {
 	TXT_INVALID_CHARSET,
 	TXT_EVAL_MAX_RECURSE,
 	TXT_PROGRAM_NOT_FOUND,
+	TXT_NO_SERVER_DEFINED,
 
 	TXT_FILL_11,