graceful fallback if the pychart lib can't be found

I'm pretty sure the (old) pychart lib is not essential to most of OpenERP server. But currently if not found it will blows up. The issue is that this restrict to use OpenERP with the CPython implementation only. I propose to fallback gracefully instead so that one coudl alst least have Jython instance, for instance to interact better with SOAP or ESB or other legacy libs at least. If that server can render the Pychart graphs, that's not a big deal, they might not be required and a second C Python instance could be launched on the same database for that purpose.