Python 3 + 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

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

Python 3 + xkcd, 55 bytes

xkcd is a third-party Python package. In Python, there is a module 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

Python 3 + 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

Python 3 + xkcd, 55 bytes

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

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

For 404: urllib2.HTTPError: HTTP Error 404: Not Found

For 859: IndexError: list index out of range

Python 3 + xkcd, 55 bytes

xkcd is a third-party Python package. In Python, there is a module 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