Use assertRaisesRegexp

[davidalber]

A number of tests are written to verify an expected exception is raised and that its message matches what is expected. These tests are currently using assertRaises and a subsequent try-except block. Example:

self.assertRaises(DuplicateNodeError, self.graph1.add_node,
                  "Leonhard Euler", "Universitaet Basel",
                  1726, 38586, set(), set())

try:
    self.graph1.add_node("Leonhard Euler", "Universitaet Basel",
                         1726, 38586, set(), set())
except DuplicateNodeError as e:
    self.assertEqual(str(e),
                     "node with id {} already exists".format(38586))
else:
    self.fail()

This can be improved by using assertRaisesRegexp.