Avoid traceback in log when httplib2 fails import

[Ryan J Ollos]

I see the following in the logs,

2015-03-26 20:19:26,045 Trac[loader] ERROR: Skipping "spamfilter.captcha.mollom = tracspamfilter.captcha.mollom": 
Traceback (most recent call last):
  File "/path/to/lib/python2.6/site-packages/Trac-1.0.5-py2.6.egg/trac/loader.py", line 68, in _load_eggs
    entry.load(require=True)
  File "/path/to/pve/lib/python2.6/site-packages/distribute-0.6.14-py2.6.egg/pkg_resources.py", line 1954, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/path/to/pve/lib/python2.6/site-packages/TracSpamFilter-1.0.6dev_r13842-py2.6.egg/tracspamfilter/captcha/mollom.py", line 23, in <module>
    from tracspamfilter.filters.mollom import MollomFilterStrategy
  File "/path/to/pve/lib/python2.6/site-packages/TracSpamFilter-1.0.6dev_r13842-py2.6.egg/tracspamfilter/filters/mollom.py", line 16, in <module>
    import httplib2
ImportError: No module named httplib2

We should trap the ImportError to avoid a traceback in the logs. I'll prepare a patch later.

[Dirk Stöcker]

In r13941.