A missing 'isbn'.cover file in an Alexandria library directory causes the exporter to crash. When I try to export, I
see an error message:

"Export failed

No such file or directory - /home/kip/.alexandria/My Library/-1191581792.cover"

The bug is likely related the lack of the cover image file and the exporter not checking for that condition and automatically
substituting the no-cover default.

On a side-note, there is likely some magic with the use of the 

   -isbn.yaml

filename encoding because some entries have the hyphen and some do not, but I don't see a pattern with the limited samples
I've seen and haven't reviewed the code or documentation to puzzle it out. I have many such examples though, such as...


/home/kip/.alexandria/My Library/-119253182.yaml
/home/kip/.alexandria/My Library/-641481208.yaml
/home/kip/.alexandria/My Library/-1172515871.yaml
/home/kip/.alexandria/My Library/-1191581792.yaml
/home/kip/.alexandria/My Library/-1576400952.yaml
/home/kip/.alexandria/My Library/-1580142001.yaml
/home/kip/.alexandria/My Library/-1651969483.yaml
/home/kip/.alexandria/My Library/-1703387209.yaml
/home/kip/.alexandria/My Library/-1730803635.yaml
/home/kip/.alexandria/My Library/-1777897005.yaml
/home/kip/.alexandria/My Library/189029268.yaml
/home/kip/.alexandria/My Library/354843309.yaml
/home/kip/.alexandria/My Library/440649378.yaml
...

It would probably be a good idea to provide a configuration option that allows the user to create the 'no-cover' default
image to be used whenever a cover image isn't available.


I am using 0.6.6 under Ubuntu Maverick on amd64, in case that helps.