Fix random errors due to previous errors managed by the error handler

Funkload tests show that when an error occurs (trying to access to a document that does not exist for instance), following valid calls will end up with an error.
Symptoms show that the conversation is lost.

Problem can also be reproduced by hand even if it's not so straight-forward (does not happen all the time).
Steps to reproduce:

• type a non-existing url => error page
• use the 'back' button of browser and click on a document link
  => error (and url has lost document context)