Windows: afslogon make error dialog system modal

Windows: afslogon make error dialog system modal

Prevent error dialogs from getting lost behind the desktop
during logon.  Make them system modal.  Add a warning icon
as well.

Reviewed-on: http://gerrit.openafs.org/7630
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit ae5aba17cc5d45805f7d263fd645fba0d3bb5116)

Change-Id: If5031c21b008306f1179cb460b39fc7d3662fbd0
Reviewed-on: http://gerrit.openafs.org/8630
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>