Since 5425316eff errors when signing up will only display two
messages:
* "Another user is already registered using this email address." or
* "Could not create a new account."
While Odoo creates a multitude of other comprehensible error messages
such as
* "Passwords do not match; please retype them."
* "Signup token '%s' is no longer valid"
This commit now separate UserError and AssertionError from SignupErrors.
Those are still hidden in a general message (see 5425316eff for reasons) while
the other ones are fully displayed.
This commit also improves translations of messages.
Denis Vermylen
676022e3af