bug fix around latest skip-bad-book feature

bug fixes around new skip-bad-book feature

Truncate too-long error message

When there's a problem downloading a book, you get the option to skip…

… the file temporarily or permanently. This can be useful with extremely old audible titles where the modern download may no longer be supported

If no codec during download, retry with all library flags enabled

increase version

Better error logging when codecs are not known

bug fix: null ref exception

New in v4.1.0

- upgrade all to .NET5
- bug fix: when codec doesn't appear in prioritized list, just get the 1st available
- add more account privacy in logs

bug fix: include new ApprovalNeeded in enumeration singletons