Explicit error when cracker exe is missing

The ProjectCracker exe must be adjacent to the dll for it to shell to.
When it's not an unclear Win32Exception is given. This explicitly checks
and says what file is missing.

Author: cgravill

src/fsharp/FSharp.Compiler.Service.ProjectCracker/ProjectCracker.fs

@@ -48,8 +48,10 @@ type ProjectCracker =
             arguments.Append(' ').Append(k).Append(' ').Append(v) |> ignore
         let codebase = Path.GetDirectoryName(Uri(typeof<ProjectCracker>.Assembly.CodeBase).LocalPath)
 
+        let crackerFilename = Path.Combine(codebase,"FSharp.Compiler.Service.ProjectCrackerTool.exe")
+        if not (File.Exists crackerFilename) then failwithf "ProjectCracker exe not found at: %s it must be next to the ProjectCracker dll." crackerFilename
         let p = new System.Diagnostics.Process()
-        p.StartInfo.FileName <- Path.Combine(codebase,"FSharp.Compiler.Service.ProjectCrackerTool.exe")
+        p.StartInfo.FileName <- crackerFilename
         p.StartInfo.Arguments <- arguments.ToString()
         p.StartInfo.UseShellExecute <- false
         p.StartInfo.CreateNoWindow <- true