The notifications center logs exceptions thrown's messages,
and to do so pass the exception to the log method.
As this method wants a string as parameter, it was converted as string.
But as we plan to use declare(strict_types=1), such implicit conver-
sion isn't welcome.
Fixes the following issue detected by phan:
app/Exceptions/Handler.php:49 PhanTypeMismatchArgument Argument 1 (message) is \Exception|\Throwable but \Psr\Log\LoggerInterface::error() takes string defined at vendor/psr/log/Psr/Log/LoggerInterface.php:66