GH2087: log nested aggregate exceptions

cake-build#2087

When the exception thrown in a task is actually a nested `AggregateException`, the output log is:

```
An error occurred when executing task 'taskName'.
Error: One or more errors occurred.
	One or more errors occurred.
```

The first occurence of `One or more errors occurred.` is the message of the parent `AggregateException`. The second occurence is the message if the inner exception, which is itself another `AggregateException`.

Here we add a call to `.Flatten()`. This creates a new single instance of `AggregateException` without nesting, so that the last log line will now show the actual inner exception message.

Reference: https://docs.microsoft.com/en-us/dotnet/api/system.aggregateexception.flatten?view=netframework-4.7.2

src/Cake/Program.cs

@@ -100,7 +100,7 @@ private static int LogException<T>(ICakeLog log, T ex) where T : Exception
  var aex = ex as AggregateException;
  if (aex != null)
  {
- foreach (var exception in aex.InnerExceptions)
+ foreach (var exception in aex.Flatten().InnerExceptions)
  {
  log.Error("\t{0}", exception.Message);
  }