Write an Appendix on how a consumer decides if a run has failed

[ghost]

@michaelcfanning suggested we add an Appendix that explains how a consumer can look at a log file and decide that the results are useful and complete. Some possible answers are:

• invocation.toolExecutionSuccessful is true for all elements of run.invocations,
• There are no toolExecutionNotifications or toolConfigurationNotifications in any invocation object whose level is "error",
• run.results is non-null.

Actually that's a pretty darned short appendix. Perhaps I can find another place for this useful information.