Update UserInput.cs: if Grasshopper, RhinoCommon or other tools cannot be found

Bugfix: if Grasshopper, RhinoCommon or other tools cannot be found, then nothing should happen. This should fix at least an error for some people
Merge pull request #1 from piac/patch-1

Update UserInput.cs: if Grasshopper, RhinoCommon or other tools cannot b...