def add_checker(checker = nil, &block)
      if block_given?
        @error_checkers << block
      elsif Proc === checker
        @error_checkers << checker
      else
        raise ArgumentError, "argument must be a Proc or block"
      end
    end
def add_checker(checker = nil, &block)
      if block_given?
        @error_checkers << block
      elsif Proc === checker
        @error_checkers << checker
      else
        raise ArgumentError, "argument must be a Proc or block"
      end
    end
Add a 'checker' proc that will be run on every page load

 @param [Proc] checker The proc to be run (can also be given as a block)
 @yieldparam [Celerity::Browser] browser The current browser object.
 @raise [ArgumentError] if no Proc or block was given.