handle utf8 in errorlogs

[mek-apelsin]

WHEN printing error messages
AND python2 hasn't figured out if stdout handles utf8 and fallbacks to ascii
AND the "source" of an image is a char over 127:

This line cute little thing:

print >>warn, "Could not load image: %s" % src

makes the feed fail.

[mek-apelsin]

It's easy to reproduce by running rss2email with cron, on a computer with python2.7 as default and insert the character 'ß' as the source of an image in file:///path/to/test.rss

[rcarmo]

Thanks. It'll be a while before I can take a look at this, but I'll take a pull request if you manage a fix.