[FIX] report: don't hide errors in custom reports
[FIX] report: don't hide errors in custom reports

If a custom report's render_html raises a KeyError for some
reason, Report.get_html would swallow the exception and fallback
on generic HTML rendering, usually blowing up with unfathomable
errors of missing rendering context data.

Check if a custom report object exists instead of waiting for a
KeyError.
ygol/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,dfang/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo