Overhaul exception reporting.

[rbtcollins]

unittest2 recently added the ability to show local variables in
tracebacks as #111
requested for us. Reusing that requires some refactoring of our code,
in particular where we were reimplementing bits of the traceback
module. Now we can just hard-depend on traceback2 and linecache2 which
are brought in by unittest2 1.0.0.

Change-Id: Ieb3268029d26b48ed4fcd25ed644bd339f6aa3fb

[jml]

I'd call out the new dependency on traceback2 as a separate bullet point (although, thank heaven that deps aren't quite as much of a pain these days).

Also, nitpick, AIUI, it's not they can be used instead, but rather that we are using them. Is that right?

[jml]

The pypy3 tests fail:

======================================================================
FAIL: testtools.tests.test_testresult.TestTextTestResult.test_stopTestRun_current_time
----------------------------------------------------------------------
Traceback (most recent call last):
  File "testtools/tests/test_testresult.py", line 1388, in test_stopTestRun_current_time
    DocTestMatches("... in 0.001s\n...", doctest.ELLIPSIS))
  File "testtools/testcase.py", line 435, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: Expected:
    ... in 0.001s
    ...
Got:
    <BLANKLINE>
    Ran 1 test in 0.000s
    OK

[rbtcollins]

The test failure is not related to this code - note the 0.0000s vs 0.0001s.

[thomir]

Ugh - this nested try / except is starting to look silly. I guess for now it's OK though. Let's hope we don't find a test runner that accepts 'tb_locals' but not 'verbosity :D

[thomir]

LGTM - can't wait to have locals printed in testtools :D

Cheers,

[rbtcollins]

I'm going to merge this now as it has Thomi's +1 and Jono had no API concerns in his feedback (and I addressed all the please-do-X stuff).