Merge branch 'add_tests' into database_direct_fixes-add_tests
raise PyslidException instead of returning nothing
raise PyslidException on errors
features.has should return a bool except for unexpected exceptions
Revert "Removed all references to PyslidException for now"

This reverts commit a6c2bfadd960ad3cdc51c65775b33e88e17280ad.
Added pyslid.database.direct.removeDuplicates to remove duplicate entries
 in ContentDB.