For some reason that we have not yet determined, the overlay that we use
for the drivers code sometimes fails to unmount with EBUSY.
When it happens, the _list_drivers task would fail, making subsequent
GET calls to /drivers?wait=true fail as well and produce an error
report.
We now mark failures in the overlay unmount operation as non fatal.
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>
6954b309e6