Fix the following warning and turn on "treat warnings as errors" for our .NET projects.
```
Deployment/InvokeOptions.cs(6,60): warning CS1574: XML comment has cref attribute 'InvokeAsync{T}(string,
```
After creating an initial Pulumi .NET project, it will be natural for some folks (who are unfamiliar with Pulumi) to try to run it via `dotnet run`, as that's how you'd typically run a .NET Core program. Doing so today fails with:
```
Unhandled exception. System.InvalidOperationException: Environment did not contain: PULUMI_MONITOR
at Pulumi.Deployment..ctor()
at Pulumi.Deployment.RunAsync(Func`1 func)
at Pulumi.Deployment.RunAsync(Func`1 func)
at Pulumi.Deployment.RunAsync(Action action)
at Program.Main() in /Users/user/temp/quickstart/Program.cs:line 9
at Program.<Main>()
```
Instead, provide a more descriptive error message indicating that the pulumi CLI should be used to run the program. We return the same error as we do for Node.js and Python.
```
Unhandled exception. System.InvalidOperationException: Program run without the Pulumi engine available; re-run using the `pulumi` CLI
at Pulumi.Deployment..ctor()
at Pulumi.Deployment.RunAsync(Func`1 func)
at Pulumi.Deployment.RunAsync(Func`1 func)
at Pulumi.Deployment.RunAsync(Action action)
at Program.Main() in /Users/user/temp/quickstart/Program.cs:line 9
at Program.<Main>()
```
