Update test reports to show all exceptions thrown in flutter

[karolinaoparczyk]

Issue: Test reports don't contain useful info ("Multiple exceptions were thrown") #951

The first version (without calling previousOnError in our onError) didn't work with Patrol's finders. A test tapping on a widget that is not on a screen would hang without any logs instead of failing (maybe it has something to do with using async guards, not sure).
Right now (with calling previousOnError in our onError) we don't catch all errors in onError, but we do avoid the Multiple exceptions were thrown message.

Because we're intercepting the errors in onError, overriding reportTestException is not needed. Moreover, it would collect the Multiple exceptions were thrown message that can be created when we call previousOnError ourselves.
Probably the exceptions are thrown so close to each other, that us calling previousOnError makes them go through its code and we miss them.

Original onError saves the error in a variable and when another one comes, it saves Multiple exceptions were thrown message to this variable. There is a method takeException() that assigns null to this variable, but trying to do it in our onError before calling previousOnError doesn't work as expected (we do not take it out, so that the Multiple exceptions were thrown is not saved and dumped to console).

The conclusion is that we successfully show real exception descriptions instead of a general Multiple exceptions were thrown message. Although we get only two out of four known exceptions that should be caught in overflow_test.dart, this is still progress. The exceptions shown are the first ones to be thrown.

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