Unwrap aggregate exceptions before watson

src/Compilers/Core/Portable/InternalUtilities/FailFast.cs

@@ -18,6 +18,13 @@ internal static void OnFatalException(Exception exception)
                 Debugger.Break();
             }
 
+            // don't fail fast with an aggregate exception that is masking true exception
+            var aggregate = exception as AggregateException;
+            if (aggregate != null && aggregate.InnerExceptions.Count == 1)
+            {
+                exception = aggregate.InnerExceptions[0];
+            }
+
             Environment.FailFast(exception.ToString(), exception);
         }