Thread:Support/About MediaWiki:Maintenance-createAndPromote-error-password/en/reply

$1 is the password error message, digging through the maintenance script itself the possible messages seem to be (message keys):
 * password-change-forbidden
 * passwordtooshort
 * password-name-match
 * password-login-forbidden
 * externaldberror
 * Anything returned by the isValidPassword hook.