fix problem with doc database in read-only mode

Now that the documentation-build phase of `raco setup` uses read-only
mode, contention between writing and reading sometimes (though
infrequently) triggers an SQLITE_IOERR_LOCK error. Change the SQLite
binding to expose that specific error, as well as SQLITE_IOERR_BLOCKED,
because a retry of the affected operation is appropriate.