How to fix “couldn't connect to host” error installing Unity Webapps?

Question

I get the following errors when I try to install Unity Webapp on Ubuntu 12.04:

root@detro-X101H:/home/detro# sudo add-apt-repository ppa:webapps/preview
Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 125, in <module>
    ppa_info = get_ppa_info_from_lp(user, ppa_name)
  File "/usr/lib/python2.7/dist-packages/softwareproperties/ppa.py", line 80, in get_ppa_info_from_lp
    curl.perform()
pycurl.error: (7, "couldn't connect to host")

My Internet connection seems to be working fine otherwise. I think I am using a proxy server. When I run apt-get update as root, I get these error messages (full output here):

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F1773AF13B1510FD
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8CD60EC948894010
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A45934EA9D4C08B
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C2518248EEA14886
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 32B18A1260D8DA0B
W: GPG error: https://private-ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E131728675254D99
W: Failed to fetch http://ppa.launchpad.net/launchpad.net/~scopes-packagers/ubuntu/dists/precise/main/source/Sources  404  Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.

Here's the ouput of cat /etc/apt/sources.list; for X in /etc/apt/sources.list.d/*; do echo; echo; echo "** $X:"; echo; cat $X; done (which lists all enabled software sources).

I'm new to Linux, and I will provided more details as needed.