[MM-16149] Fix username validation message for Bot creation. (matterm…

…ost#2988)

* check username validation with utils function.  Otherwise, error is
handled server-side with actually does not enforce the a username to
start with [a-z]

* refactor.  Catch usernameError before we even attempt server-side bot
creation call

components/integrations/bots/add_bot/add_bot.jsx

@@ -277,6 +277,15 @@ export default class AddBot extends React.Component {
  return;
  }
  } else {
+ const usernameError = Utils.isValidUsername(bot.username);
+ if (usernameError) {
+ this.setState({
+ adding: false,
+ error: usernameError,
+ });
+ return;
+ }
+
  const result = await this.props.actions.createBot(bot);
  if (result) {
  data = result.data;