Do not import exceptions of requests / load browser if needed

The import process of requests is very expensive: it imports all urllib,
and a lot of stuff. It is a problem on slow hardware, for applications
not running browser2 (example: weboob-config update).

This patch has two mains parts:
 * introduces load_browser to load a browser only if needed (a lot of repository
   processing does not need a browser)
 * Do not import exceptions of requests, but use Weboob internals one

Before: 6455 open/190 modules imported
After: 3997 open/112 modules