fix exception message in report seachr

when we get to the raise statement, `r` is `None`, so displaying it in the
exception message is useless. `name` is what we want to show.
Merge pull request #2796 from gurneyalex/8.0-improve_report_not_found_exception

[FIX] lookup_report: exception message when the report is not found
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64