You can capture a screenshot on test failure but i don't think it would help to get a screenshot at app crash. What would this serve in this context?
I believe you would get a main screen on iOS and an 'unfortunately' message in android?
I am not sure about iOS but on Android i used to get live logcat output equal and higher to ERROR level than check them if there is anything FATAL, if there is report it to certain places. (This being done in test scripts, by a helper method)