Clean up log.err calls, for one of the issues in #889.

Clean up log.err calls, for one of the issues in #889.

allmydata.util.log.err() either takes a Failure as the first positional
argument, or takes no positional arguments and must be invoked in an
exception handler. Fixed its signature to match both foolscap.logging.log.err
and twisted.python.log.err . Included a brief unit test.