Clarify exception message for tests that fail due to a prior exception

Fixes #124

src/Microsoft.VisualStudio.Extensibility.Testing.Xunit.Shared/Threading/ErrorReportingIdeTestRunner.cs

@@ -27,7 +27,7 @@ protected override Task<decimal> InvokeTestMethodAsync(ExceptionAggregator aggre
                 () =>
                 {
                     var tcs = new TaskCompletionSource<decimal>();
-                    tcs.SetException(_exception);
+                    tcs.SetException(new InvalidOperationException("Test execution was skipped due to a prior exception in the harness.", _exception));
                     return tcs.Task;
                 });
         }