Because your stack trace should be more encouraging

C# Web Request

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
Amazon.Runtime.Internal.Unmarshaller+<InvokeAsync>d__3`1.MoveNext()
  File "System.Runtime.ExceptionServices.ExceptionDispatchInfo", line 12, in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
  File "System.Runtime.CompilerServices.TaskAwaiter", line 46, in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task task)
  File "System.Runtime.CompilerServices.TaskAwaiter", line 14, in System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(System.Threading.Tasks.Task task)
  File "D:\JenkinsWorkspaces\trebuchet-stage-release\AWSDotNetPublic\sdk\src\Core\Amazon.Runtime\Pipeline\ErrorHandler\ErrorHandler.cs", line 99, in Amazon.Runtime.Internal.ErrorHandler+<InvokeAsync>d__5`1.MoveNext()
System.Net.WebException: The request was aborted: The request was canceled.Traceback (most recent call last):
  File "System.Net.ConnectStream", line 426, in System.Net.ConnectStream.CloseInternal(System.Boolean internalCall, System.Boolean aborting)
System.IO.IOException: Cannot close stream until all bytes are written.

Did you check the bathrooms?
Next time check the bathrooms. 
All customers must be gone before we close the store.