Before this commit, if the loading of a js or a css asset would fail
(via useAssets), a traceback with an incorrect message would appear.
This was due to the fact that a promise was rejected with an event as
parameter. We correct this by rejecting the promise with a custom error.
closesodoo-dev/odoo#960
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>