[FIX] web: show error when trying to export a report with more than 256 columns
[FIX] web: show error when trying to export a report with more than 256 columns

If the user tried to generate a report with more than 256 columns,
he would get a traceback instead of an error message.
Furthermore, if only one measure was selected, then the check was not done
and the user would also get a traceback.

opw-1816931

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo