Don't retry too fast to fetch icon after failure
I have an icon field for which the filename is not good. i.e. the server cannot
find the file.
I do not know why, but it looks like that the error is not reported properly.
Furthermore, the thread stays open server side.
Using the application (with the client) creates more and more of those threads,
until the os fails.
How to reproduct :
- Install client, server from mercurial
- Create new database
- rename ir/ui/icons/tryton-list.svg to something else
- connect to the server using the client
- Thread number should increase as soon as the client tries to load the failing
icons (go to Admin => UI => icons, open the list and refresh to trigger a