Feature Request

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

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.