assertRaises gives error about a lambda

[Jonathan Lange]

======================================================================
FAIL: tests.test_database.TestDatabase.test_double_insert
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jml/src/pkgme-binary/symbol-database/devportalbinary/tests/test_database.py", line 38, in test_double_insert
    db.insert_new_library, 'libfoo.so.0', 'foo')
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 388, in assertRaises
    self.assertThat(lambda: callableObj(*args, **kwargs), matcher)
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 411, in assertThat
    raise MismatchError(matchee, matcher, mismatch, verbose)
MismatchError: <function <lambda> at 0x365ff50> returned None
id=65, tests=46, failures=1

The MismatchError would be more helpful if it said "db.insert_new_library" or "insert_new_library", rather than a lambda that testtools made up.

[Robert Collins (lifeless)]

I think we could sanely poke inside the lambda to get at the function it calls; that may sound invasive but will:
 - make our rendering nicer
 - handle lambda supplied *to us* more nicely as well.

[Jonathan Lange (jml)]

I've taken a different approach in my proposed patch.