[merp] Capture Environment.FailFast message in crash report (#18863)
* [merp] Capture Environment.FailFast message in crash report
* [test] merp-crash-test refactor crasher data; add optional validator
Switch from a Tuple<T1,T2> to a struct for the crash items. Add a third
optional validator field which is an action to run on the json blob that we get
back.
Not updating existing tests to do any json validation, yet.
* [test] Add new MERP crash test for Environment.FailFast
Check that the resulting json blob includes the failfast message.