Fix '403 Forbidden. You should know better.' on missing User-Agent

Hello,

For some reason, Drupal.org administrators has decided to block http clients without a User-Agent.[1] Broken as this is, it means that dh-make-drupal cannot fetch the files needed to build packages.

There are two problems that are actually adressed by my changes:
a) The actual error is hidden. The user cannot know if it is 403 or 404 error or something else. I fixed this by printing contents also of $! on related exceptions.
b) Since Drupal.org now requires

Thanks,
Stefan Kangas

    [1] See discussion at https://drupal.org/node/1916598