don't mask missing file errors when trying to read the config

I ran into a problem getting "Did not find config file: config/solo.rb" when it was definitely there, and after poking around in the code realized what was happening - within a library called from config/solo.rb, I was trying to read another file, which was missing. However the rescue block for reading the config assumes Errno::ENOENT means the config file itself was missing

This fix is a bit hack-ish - there's probably a bunch of better solutions, but I wanted to point out the problem and provide at least a minimal fix