No errors when package is not found

[MarcBruins]

This PR includes 2 fixes.

1. NuKeeper throws if the branch already exists. This shouldn't happen in my opinion. If you are to lazy to "accept" the PR's then that should be fine. It is also optional in Azure DevOps to delete the branch after a merge. If you keep the branch then NuKeeper will stop working because it detect a stale old branch(shouldn't happen that much because it's bound to a specific upgrade but still).

The RunFinderForSource gets called for every nuget source that you have. If it can't authenticate for a specific source that shouldn't be a problem because it can still succeeded, the error create the perception that it fails.

2. For the project i'm working on right now we don't want any secrets in our code. That means that for some project in our solution we include a nuget.config that has credentials that looks like this:

  <packageSourceCredentials>
    <VSTS>
      <add key="Username" value="dontcare@dontcare.com" />      
      <add key="ClearTextPassword" value="__System.AccessToken__" />
    </VSTS>
  </packageSourceCredentials>

The System.AccessToken is injected by our CI. However NuKeeper will use this for it's command. The tool will still work fine because on our machine we have a system wide nuget.config which contains a PAT. The downside is that any command generates a hell of a lot Errors and our whole screen is almost red.

[AnthonySteele]

The branch part addresses #624 I think.