Errors raised during the generation process are not propagated correctly to the build host

More generally, any error caused by running custom template code will be thrown with the following unhelpful message:

```
 Generators failed with 2 error(s) (6.2s) → Generators\bin\net6.0\Generators.dll
    EXEC : error : Error running transform: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
```