rml2pdf: fix logging of exceptions.
rml2pdf: fix logging of exceptions.

exception() cannot be called without a string, yet it is even better to
demote those logs to warnings.

Conflicts:

	bin/report/render/rml2pdf/utils.py

gisce/openobject-server,gisce/openobject-server,vnc-biz/openerp-server,MarkusTeufelberger/openobject-server,MarkusTeufelberger/openobject-server,gisce/openobject-server,MarkusTeufelberger/openobject-server,vnc-biz/openerp-server,ovnicraft/openerp-server,splbio/openobject-server,splbio/openobject-server,splbio/openobject-server,ovnicraft/openerp-server