OwnSql: Distinguish no-data from error #6677

OwnSql: Distinguish no-data from error #6677

This could fix a problem where the client incorrectly decides to delete
local data.

Previously any sqlite3_step() return value that wasn't SQLITE_ROW would
be interpreted as "there's no more data here". Thus an sqlite error at a
bad time could cause the remote discovery to fail to read an unchanged
subtree from the database. These files would then be deleted locally.

With this change sqlite errors from sqlite3_step are detected and
logged. For the particular case of SyncJournalDb::getFilesBelowPath()
the error will now be propagated and the sync run will fail instead of
performing spurious deletes.

Note that many other database functions still don't distinguish
not-found from error cases. Most of them won't have as severe effects on
affected sync runs though.