uci: handle deleting of uci options gracefully
Currently, trying to delete uci option might raise UciException
if option doesn't exist.
To handle this, there is a lot of blocks like:
try:
backend.del_option(...)
except UciException:
pass
It would be nice to get rid of this boilerplate code and use something like this instead:
backend.del_option(..., fail_on_error=False)