Use innerException for Task.AsUniTask

[hadashiA]

#422 #428

The problem is that if Task.AsUniTask throws an exception, it becomes an AggregateException.

It seems that there is a historical reason why Task holds AggregateException internally.
We have fixed it as such, as supplementing InnerException is probably sufficient in most cases.

[hadashiA]

Even if there are multiple InnerExceptions, always unwrapping.
This is because the behavior is the same as await Task.WhenAll().
Is this a problem?

[hadashiA]

However, it is more natural to get multiple exceptions when WhenAll.
I ended up going for the same behavior as await Task.WhenAll, but maybe I should break it and still provide AggregateException.
Sorry, but I would like to discuss this a bit.

[neuecc]

var t1 = Task.Run(() => 100);
var t2 = Task.Run(async () => { await Task.Delay(1000); throw new Exception("E1"); });
var t3 = Task.Run(async () => { await Task.Delay(5000); throw new Exception("E2"); });

var t = Task.WhenAll(t1, t2, t3);

try
{
    // await WhenAll wait all result(even if contains error)
    await t;
}
catch (Exception ex)
{
    // show first Exception(E1)
    Console.WriteLine(ex);
}

Console.WriteLine("----");

try
{
    // same as .Exception
    t.Wait();
}
catch (Exception ex)
{
    // show AggregateException(E1, E2)
    Console.WriteLine(ex);
}

when access .Exception, Task create AggrgateException from holded exceptions. https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/TaskExceptionHolder.cs,252

when await Task and if the task faulted, throw its first exception, even if it contained more than one.
https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/TaskAwaiter.cs,148

--

This PR implementation (throw an InnerException for one and an AggregateException for more than one) is OK.