UBUNTU: SAUCE: integrity: downgrade error to warning

UBUNTU: SAUCE: integrity: downgrade error to warning

BugLink: https://bugs.launchpad.net/bugs/1766201
In 58441dc86d7b the error "Unable to open file: ..." has been downgraded
to warning in the integrity/ima subsystem. Do the same for a similar
error message in the generic integrity subsystem.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>