Improve display failure if thumbnails time out
Improve display failure if thumbnails time out

In Debug mode we display a message box through Debug.Fail if creation of
thumbnails time out. It should be sufficient to do that only once and on
the UI thread, otherwise the application might become unusable.

JohnThomson/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,JohnThomson/BloomDesktop,StephenMcConnel/BloomDesktop,gmartin7/myBloomFork,JohnThomson/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,BloomBooks/BloomDesktop,andrew-polk/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,andrew-polk/BloomDesktop