def setFailAction( self, fn ):
self.failAction = fn<EOL>return self<EOL>
Define action to perform if parsing fails at this expression.
           Fail acton fn is a callable function that takes the arguments
           C{fn(s,loc,expr,err)} where:
            - s = string being parsed
            - loc = location where expression match was attempted and failed
            - expr = the parse expression that failed
            - err = the exception thrown
           The function returns no value.  It may throw C{L{ParseFatalException}}
           if it is desired to stop parsing immediately.