Missing handling of "close" errors

When backing up on a CIFS target (others?), a full (remote) file system
is not detected until close() returns (returning -1 and setting errno).
However, the return value is not verified, and the incorrect write
operation is not detected. The final job result is "ok", but the volume
files are small or empty.

Fixes #233: Missing handling of "close" errors

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>