Locate OmniSharp launch executable more robustly

[DustinCampbell]

This is a follow-up work item to PR #153. That change addressed a problem where searching for the OmniSharp launcher was failing on Linux because we assumed that the underlying system was case-insensitive. We had fixed it by simply changing the case of the executable that we searched for, but this is not necessarily robust against future OmniSharp changes.

[gregg-miskelly]

BTW: 'fnd' is spelled wrong in that error message too.

[DustinCampbell]

thanks Gregg!