actions/image_partition: do not return error if removing buildtime mountpoint with a read-only parent
actions/image_partition: do not return error if removing buildtime mountpoint with a read-only parent

Returning an error in this function causes the rest of the cleanup operation to
not be executed; so in this case downgrading an error to remove the mountpoint
from the read-only filesystem to a warning is wise.

Resolves: #206

Signed-off-by: Christopher Obbard <0becaf7888cff31ba54e40b1ffb8263e09b8b7da@collabora.com>

go-debos/debos