Fix #25
This adds an option to silently ignore failed conversions and remove
them from the list of books to upload. At the end, any failures are
printed to the job's details (view the job list, select the job, click
the button to show job details). The default for this is "off", which
will cause the first failed conversion to raise an exception. Exceptions
are all trapped and so the notice displayed should now always include
the book title and authors.

Although it isn't possible to have each book processed and uploaded
separately, this change should address both parts of this enhancement
request and make it more useful regardless of whether the silent ignore
feature is chosen or not.