#43 thumbnail generation error recovery

[Joe Romaine]

If the image server encounters an exception (or error
for that matter) while generating a thumbnail, and was
half way through writing the file out, a garbled file
is left in the thumbnail folder. It would be ideal if
the file was written to a temporary location, then
moved (i.e. piped) to the appropriate location when all
has gone well. This will reduce the amount of garbled
thumbnails that are generated.