'Honour the longMessage attribute when generating failure messages.
If longMessage is False this means:
* Use only an explicit message if it is provided
* Otherwise use the standard message for the assert
If longMessage is True:
* Use the standard message
* If an explicit message is provided, plus \' : \' and the explicit message'

def _formatMessage(self, msg, standardMsg):

    if (not self.longMessage):
        return (msg or standardMsg)
    if (msg is None):
        return standardMsg
    return ((standardMsg + '    :    ') + msg)
