Cannot set TFM through DTE

In previous version of the project system you could set the TFM for a project directly on a DTE project object like so:
```C#
project.Properties.Item("TargetFrameworkMoniker").Value = ".NETFramework,Version=v4.6.2";
```

If you attempt this in the new project system today you will get an error:
```C#
project.Properties.Item("TargetFrameworkMoniker").Value = ".NETStandard,Version=v1.5";
```
```
System.InvalidOperationException : Can't set the value of a read-only property.
```

We should either make this scenario work or throw an error stating that setting the TFM is not supported instead of a generic exception.