unit tests of buildbot.status.web.base.plural() method
unit tests of buildbot.status.web.base.abbreviate_age methid()
unit test config.ConfigErrors.__str__() method

Added unit test on config.ConfigErrors.__str__() method
unit test configure file open error in MasterConfig.loadConfig()

Added a unit test that simulates IOError when opening configure file in
config.MasterConfig.loadConfig(). Check that the expected ConfigErrors
exception is rised.
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.