Fixed exception when importing bloom.logging

There is incorrect exception-handling code in `bloom.logging` that throws an exception as soon as it is imported. This is a very annoying problem since `bloom.logging` is included in all of the command-line scripts.

The error looks like this:

    $ bloom-generate 
    Traceback (most recent call last):
      File "/usr/local/bin/bloom-generate", line 9, in <module>
        load_entry_point('bloom==0.5.12', 'console_scripts', 'bloom-generate')()
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 339, in load_entry_point
        return get_distribution(dist).load_entry_point(group, name)
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 2457, in load_entry_point
        return ep.load()
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 2171, in load
        ['__name__'])
      File "/usr/local/lib/python2.7/dist-packages/bloom/commands/__init__.py", line 42, in <module>
        from bloom.commands.update import start_updater
      File "/usr/local/lib/python2.7/dist-packages/bloom/commands/update.py", line 49, in <module>
        from bloom.logging import warning
      File "/usr/local/lib/python2.7/dist-packages/bloom/logging.py", line 303, in <module>
        debug(str(exc.__name__) + ": " + str(exc))

This caused by a bug in how the error message is constructed: it uses `__name__` instead of `__class__.__name__`. This only happens if writing a file log fails, e.g. because the logging directory is not writable.