Feature: custom exception for session configuration errors

[jvanasco]

Feature Request

When there is a configuration issue, such as no session factory configured, Pyramid raises a standard AttributeError. See:

pyramid/src/pyramid/request.py

Lines 189 to 201 in 68a6cb1

	@reify
	def session(self):
	"""Obtain the :term:`session` object associated with this
	request. If a :term:`session factory` has not been registered
	during application configuration, a
	:class:`pyramid.exceptions.ConfigurationError` will be raised"""
	factory = self.registry.queryUtility(ISessionFactory)
	if factory is None:
	raise AttributeError(
	'No session factory registered '
	'(see the Sessions chapter of the Pyramid documentation)'
	)
	return factory(self)

It would be beneficial if Pyramid were to raise a custom subclass of AttributeError. It could be something as generic as this:

class PyramidConfigurationError(AttributeError):
    pass

This would be backwards compatible with existing code, but allow better control over detecting a non-configured session.

Right now, the exception string must be analyzed

try:
    session = request.session
    ...
    do things
    ...
except AttributeError as exc:
	if exc.args[0].startswith("No session factory registered"):
		do_something()
	else:
		do_something_else()

It would be preferable to catch it directly, like:

try:
    session = request.session
except PyramidConfigurationError:
	do_something()

I'm not sure how many other Pyramid components would benefit from this, so I understand the concern over its overall utility.