IRC nicks cannot contain spaces

Rather than trying and hoping the server gives a proper failure, just
fail early and avoid the whole mess. Some servers will instead send
messages to the user, etc, which is not very helpful.

src/irc/irc_client.cpp

@@ -526,6 +526,11 @@ void IrcClient::send_user_command(const std::string& username, const std::string
 
 void IrcClient::send_nick_command(const std::string& nick)
 {
+  if (nick.find(' ') != std::string::npos)
+    {
+      this->on_erroneous_nickname(IrcMessage(std::string(this->hostname), "432", {this->get_own_nick(), nick, "Erroneous nickname"}));
+      return;
+    }
   this->send_message(IrcMessage("NICK", {nick}));
 }