Ensure callers get errors when they happen

Previously we silently dropped portions or all of a block when a
duplicate quad is found. We still fail now, but we tell someone.

Fixes #201.

db/db.go

@@ -82,15 +82,21 @@ func Load(qw graph.QuadWriter, cfg *config.Config, dec quad.Unmarshaler) error {
 		block = append(block, t)
 		if len(block) == cap(block) {
 			count += len(block)
-			qw.AddQuadSet(block)
+			err := qw.AddQuadSet(block)
+			if err != nil {
+				return fmt.Errorf("db: failed to load data: %v", err)
+			}
+			block = block[:0]
 			if glog.V(2) {
 				glog.V(2).Infof("Wrote %d quads.", count)
 			}
-			block = block[:0]
 		}
 	}
 	count += len(block)
-	qw.AddQuadSet(block)
+	err := qw.AddQuadSet(block)
+	if err != nil {
+		return fmt.Errorf("db: failed to load data: %v", err)
+	}
 	if glog.V(2) {
 		glog.V(2).Infof("Wrote %d quads.", count)
 	}

writer/single.go

@@ -55,8 +55,7 @@ func (s *Single) AddQuadSet(set []quad.Quad) error {
 			Timestamp: time.Now(),
 		}
 	}
-	s.qs.ApplyDeltas(deltas)
+	return s.qs.ApplyDeltas(deltas)
-	return nil
 }
 
 func (s *Single) RemoveQuad(q quad.Quad) error {