Return bytes transferred when ReadFile fails with ERROR_MORE_DATA.

Reverted the previous change to ignore ERROR_MORE_DATA. Instead,
the error will propagated as with any other (i.e. in an error_code
or thrown as a system_error). For code that needs to handle partial
messages, the error_code overload should be used.

asio/include/asio/detail/impl/win_iocp_handle_service.ipp

diff --git a/asio/include/asio/detail/impl/win_iocp_handle_service.ipp b/asio/include/asio/detail/impl/win_iocp_handle_service.ipp
index 1281050..cf0d3a6 100644
--- a/asio/include/asio/detail/impl/win_iocp_handle_service.ipp
+++ b/asio/include/asio/detail/impl/win_iocp_handle_service.ipp
@@ -440,19 +440,16 @@
   if (!ok)
   {
     DWORD last_error = ::GetLastError();
-    if (last_error != ERROR_MORE_DATA)
+    if (last_error == ERROR_HANDLE_EOF)
     {
-      if (last_error == ERROR_HANDLE_EOF)
-      {
-        ec = asio::error::eof;
-      }
-      else
-      {
-        ec = asio::error_code(last_error,
-            asio::error::get_system_category());
-      }
+      ec = asio::error::eof;
     }
-    return 0;
+    else
+    {
+      ec = asio::error_code(last_error,
+          asio::error::get_system_category());
+    }
+    return (last_error == ERROR_MORE_DATA) ? bytes_transferred : 0;
   }
 
   ec = asio::error_code();