Added sleepycat DatabaseException to known issues.
Added sleepycat DatabaseException to known issues.

troubleshooting-guide.rst:
- Added a 'Known issues' section with a section on
  com.sleepycat.je.DatabaseException for 'http_cookies' database.

DIA-NZ/webcurator,DIA-NZ/webcurator,DIA-NZ/webcurator,DIA-NZ/webcurator