Unobserved Task Exception incorrectly causes session to end as crashed

[mattjohnsonpint]

When capturing events, we look to see if any unhandled exceptions have occurred and if so we end the session and mark it as crashed.

sentry-dotnet/src/Sentry/Internal/Hub.cs

Lines 321 to 333 in af8c42c

	var hasUnhandledException = evt.HasUnhandledException();
	if (hasUnhandledException)
	{
	// Event contains a terminal exception -> end session as crashed
	_options.LogDebug("Ending session as Crashed, due to unhandled exception.");
	actualScope.SessionUpdate = _sessionManager.EndSession(SessionEndStatus.Crashed);
	}
	else if (evt.HasException())
	{
	// Event contains a non-terminal exception -> report error
	// (this might return null if the session has already reported errors before)
	actualScope.SessionUpdate = _sessionManager.ReportError();
	}

This is a problem if the unhandled exception did not actually cause the application to crash, which happens in the case of an exception in an unobserved task.

sentry-dotnet/src/Sentry/Integrations/UnobservedTaskExceptionIntegration.cs

Lines 35 to 36 in af8c42c

	ex.Data[Mechanism.HandledKey] = false;
	ex.Data[Mechanism.MechanismKey] = "UnobservedTaskException";

We should only end the session if indeed the application is terminating.

Affects Unity also, as per discussion with @bitsandfoxes.

Relates to getsentry/rfcs#10 - but shouldn't depend on it being completed. (We can prevent the end of session whether or not we send that information to Sentry.)