Handle case where no exception is on stack.

If `logging.Logger.exception` was called without an exception being
handled anywhere on the stack (`exc_info` is `(None, None, None)`), an
exception was raised when trying to resolve `excType`. Now, `excType`
will be set to `'NoneType'` and `excValue` to `'None'`.
Fix docstrings.
Make sure excText does exist but is None.
Merge pull request #6 from mochi/no-exception

Handle case where no exception is on the stack.