Support/ignore when a project references other assemblies

So this gets Octokit back to a happy place:

 - [x] serve up the error to the logging in MSBuild - instead of just the message
 - [x] turns out the default FileResolver uses the GAC. I wrote a simple one that scans the output folder for matching assemblies
 - [x] when one cannot find an assembly, just return `null` instead of generating an exception
 - [x] when the search folder doesn't exist, ignore it

Resolves #1