local errors during web.client.downloadPage should fire errback

[warner]

If downloadPage runs into, say, an IOError("No space left on
device") while it is trying to write the page to the local
file, I'd like it to send the IOError through the deferred's
errback rather than raising an immediate exception (which, I
think, causes the HTTP fd to be dropped).

In creating a test for this, I am also planning to extend
the API for downloadPage: if the fileName argument is not a
string, then it is expected to be an open file-like object
that can handle .write (and .seek if the partialContent
stuff is enabled).

[warner]

Changed HTTPDownloader as described.

twisted/web/client.py 1.33