removed unreachable code in config.MasterConfig.loadConfig()

The call to error() with global _errors set to None, will raise
an exception. Thus following 'raise errors' is never reached.
test handling of invalid options to 'buildbot user' subcommand

Unit tests on exception code paths in
scripts.runner.UserOptions.postOptions(). Check that various
invalid options combinations for 'buildbot user' subcommand
raises the usage.UsageError exception.