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* "Signup token '%s' is no longer valid"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.
In Python 3, all of these were "consolidated" under urllib(.request,
.parse, .errors) which is inconvenient.
Since we already have hard dependencies on requests and
werkzeug(.urls, which is a backport of Python 3's unicode-aware
urllib.parse) migrate *everything* to that.
A sticking point is urllib2.URLError, those were (mostly) replaced by
the slightly more general IOError which URLError extends.
Christophe Simonis