Review DB/Data safety.

I've added the comment 'REVIEW:' where I have some concern that
the database and our copy of it could get out of step, or simply that
we fail to report a problem to the user.

I'd like these reviewed and turned into comments that make it
easier to check for correctness, for example saying where an
error is reported, or why it is OK to do no further recovery action.

src/SqliteSampleBlock.cpp

@@ -435,6 +435,7 @@ bool SqliteSampleBlock::GetSummary(float *dest,
          // Prepare and cache statement...automatically finalized at DB close
          auto stmt = Conn()->Prepare(id, sql);
          // Note GetBlob returns a size_t, not a bool
+         // REVIEW: An error in GetBlob() will throw an exception.
          GetBlob(dest,
                      floatSample,
                      stmt,