fix crochet crashing when MainThread isn't available when _shutdown module is loaded

[lieryan]

This pull request fixes a situation where this bug causes projects using crotchet with autonose to not be able to rerun changed tests because crochet cannot find the MainThread.

$ cat demo.py
from crochet import setup, run_in_reactor, TimeoutError
$ cat test_demo.py
import unittest
class Test(unittest.TestCase):
    def test_foo(self):
        import demo
$ 0launch http://gfxmonk.net/dist/0install/autonose.xml &
$ # ... wait for autonose to load and finish testing ...
$ touch demo.py

Got:

======================================================================
ERROR: test_foo (test_demo.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/path/to/project/test_demo.py", line 4, in test_foo
    import demo
  File "/path/to/project/demo.py", line 1, in <module>
    from crochet import setup, run_in_reactor, TimeoutError
  File "/path/to/env/src/crochet/crochet/__init__.py", line 25, in <module>
    from ._shutdown import _watchdog, register
  File "/path/to/env/src/crochet/crochet/_shutdown.py", line 59, in <module>
    if t.name == "MainThread"][0], _registry.run)
IndexError: list index out of range

----------------------------------------------------------------------
Ran 1 test in 0.022s

FAILED (errors=1)

Expected:

1. test reruns successfully

[itamarst]

Thanks for the patch/bug report. I'll try to get a fix in for next release.

[itamarst]

While this change does mask the problem, it doesn't seem like it solves the underlying problem. Nor is it clear to me what exactly the underlying problem is - did MainThread ever exist in this setup? Or has it already exited? Not clear. So while I do want to fix the issue it probably won't be via this PR.

[itamarst]

I opened a new issue for this, #79.