Improve error handling when complex annotations are not passed an alias

[Vinko Mlacic]

When complex annotations are not passed an alias, the default alias is usually used. If the annotate arg does not define the default alias, an error saying "Complex annotations require an alias" is raised.

This works fine with args that are subclasses of Aggregate subclass because the default_alias property on the Aggregate subclass raises that same TypeError.

However, when using expressions that are not Aggregate subclasses, this fails with an obscure message saying that default_alias is not a property on the expression object.

Code to reproduce:

models.py:

class Book(models.Model):
    name = models.CharField(max_length=255)

tests.py:

Book.objects.annotate(Value("title"))

Similar happens with other non Aggregate types like: F, Case - When, Subquery, Exists, ...