Raise an error about failing to change the dbroot value only if the directory does not exist
Raise an error about failing to change the dbroot value only if the directory does not exist

This allows targetcli to start when the dbroot value cannot be changed
because the target drivers are already registered (issue #105).

Signed-off-by: Christophe Vu-Brugier <1930e27f67e1e10d51770b88cb06d386f1aa46ae@fastmail.fm>

cvubrugier/rtslib-fb