Prevent SQL error message in case of error

`\OC\User\Database::createUser` can throw a PHP exception in case the UID is longer than
permitted in the database. This is against it's PHPDocs and we should cast this to `false`,
so that the regular error handling triggers in.

The easiest way to reproduce is on MySQL:

1. Create user `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa` in admin panel
2. Create user `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa` in admin panel again
3. See SQL exception as error message

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>