Check mono version and send an explicit error if incompatible

[mickaelistria]

Issues such as OmniSharp/omnisharp-atom#993 are pretty hard at troubleshooting. It seems like the root cause could be a too low mono version on the environment. OmniSharp should check for the version of mono or dotnet and send a more helpful error in case they are incompatible, to help users understand what they should do to get it working.

[DustinCampbell]

Ultimately, #666 should help address this.

[filipw]

there is something poetic about that particular issue being 666 😀

[DustinCampbell]

I know, right? 😀

[TheRealPiotrP]

@DustinCampbell since #666 is closed, can we close this one as well?

[DustinCampbell]

No, we still need to send information at OmniSharp start up.