TL;DR: sudo apt-get install ... saying "Unable to correct problems, you have held broken packages."

The problem

I was trying to get the WebApps feature for PP and QQ following this blog post.

I ran the sudo add-apt-repository ppa:webapps/preview command to add the repository, but i got a connection error. Since I know my current ISP gives a shaky connection, I tried again and sure enough, it worked.

Then I ran sudo apt-get install unity-webapps-preview, but I realized we had to update apt-get first, so I hit Ctrl + C to stop it. Then I ran sudo apt-get update which worked w/o a fuss, but when I ran sudo apt-get install unity-webapps-preview again later, it showed an error message. Here's the dump:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 unity-webapps-preview : Depends: xul-ext-unity but it is not going to be installed
                         Depends: xul-ext-websites-integration but it is not going to be installed
                         Depends: xul-ext-webaccounts but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I think this might be because of me interrupting the earlier command. It hadn't got a chance to output anything, though — I stopped it pretty fast.

What I tried

I have tried running a number of commands:-

• sudo apt-get install --fix-broken
• sudo apt-get autoclean
• sudo apt-get autoremove
• sudo apt-get -f install
• sudoapt-get install ppa-purgeandsudo ppa-purge ppa:webapps/preview`

Even after running sudo apt-get upgrade after every try, none of them worked.

Research

I tried searching Google, looking at a couple of forums and searching on AU, but to no avail. Help would be appreciated.