Riot should help people when they enter an incorrect username on login

[n1trux]

Description

I'd really like Riot to understand what's wrong with a username entered by the user.

Steps to reproduce

• Try to log in with an incorrect username (e.g. user:example.com, user@example.com, user@riot.example.com (web chat FQDN), user@server.example.com (home server FQDN), @user@example.com, :user@example.com)
• Try to enter the link to the riot web chat on a subdomain of your server (riot.example.com) as your home server address
• Try to enter the user domain address as your home server address if they're not the same

Riot may notify the user what could be wrong with the username or server URL they entered.

possible solution

More detailed errors when the home server isn't a home server or the integrations server isn't an integrations server.

A popup with "This looks like a link to the Riot web chat. Correct it to server.example.com?"

A popup with "This doesn't look like a valid user name. Correct it to @user:example.com?"

Riot could probe the URLs given and find out what they are.

possibly related issues

#3287, #7785, #8292, #8295, #9648, #10274