vcardstore: Check `open()` return value
vcardstore: Check `open()` return value

The directory could be read-only and it would create a silent
issue.

Change-Id: I9bff74bcaf0006d59417b0656658fc568b7c425f
Reviewed-by: Nicolas Jäger <0cb6976f4960cd47d671d91396dc19be062446b8@savoirfairelinux.com>

savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc,savoirfairelinux/ring-lrc