Add option to suppress errors when using cProfile with -o.

git-svn-id: https://mcomix.svn.sourceforge.net/svnroot/mcomix@797 6cf541bf-b722-4a7c-9fc0-fb1e75421f59
Sped up deleting library books by preventing individual db writes.

Auto-commit mode leads to harddisk access every time a single
book is deleted, making deletion of many books a rather slow operation.
This should be fixed by using a single transaction.

git-svn-id: https://mcomix.svn.sourceforge.net/svnroot/mcomix@798 6cf541bf-b722-4a7c-9fc0-fb1e75421f59