Build a Faulty XKCD Browser
Python 2.7 + xkcd,  55 bytes

xkcd is a third-party Python package. In Python, there is a package for everything!

lambda n:[xkcd.getComic(n).altText][n==859]
import xkcd



  For 404: urllib2.HTTPError: HTTP Error 404: Not Found
  
  For 859: IndexError: list index out of range