Improved error messages for undefined methods.

Whenever an undefined method is called on an object with one or more parent
definitions the error message includes the names of these parents. Consider the
following example:

    ##
    # @param [Fixnum|Bignum|Numeric] number
    #
    def double(number)
      return number.to_foo
    end

Before this commit the error message would be as following:

    undefined method to_foo on an instance of number

This commit changes this so that the error message instead becomes the
following:

    undefined method to_foo on an instance of Fixnum, Bignum or Numeric