Improve docstrings in graph flow to denote exceptions raised
Improve docstrings in graph flow to denote exceptions raised

Link the existing exceptions mentioned to there sphinx doc about
them and also adds a section about when the 'DependencyFailure'
exception is raised.

Also tweaks the class docstring a little, to make it easier to
understand.

Change-Id: Ie4b989444c5ad73660cc61c0c3b2b1702b669087

pombredanne/taskflow-1,openstack/taskflow,pombredanne/taskflow-1,jimbobhickville/taskflow,junneyang/taskflow,openstack/taskflow,jimbobhickville/taskflow,junneyang/taskflow