Improve username validation error messages and help text

When a user tries to create a username that is invalid (e.g. contains disallowed characters), the error message does not explain what made it invalid or what the rules are.

Additionally, help text has caused confusion — some users thought their email address could be used as a username.

## Requested behaviour

1. On username validation error, explain specifically why the username is invalid and what the rules are
2. Update help text to clarify that the username is not the email address

## Background

Requested in customer feedback.