Improve rowstreamer error message

Signed-off-by: Matt Lord <[email protected]>

go/vt/vttablet/tabletmanager/vdiff/workflow_differ.go

@@ -385,8 +385,8 @@ func (wd *workflowDiffer) buildPlan(dbClient binlogplayer.DBClient, filter *binl
 			return err
 		}
 
-		// We get the PK columns from the source schema as well, as they can
-		// differ and determine the proper lastPK to use when saving progress.
+		// We get the PK columns from the source schema as well as they can differ
+		// and they determine the proper position to use when saving our progress.
 		if err := td.getSourcePKCols(); err != nil {
 			return err
 		}

go/vt/vttablet/tabletserver/vstreamer/rowstreamer.go

@@ -276,7 +276,8 @@ func (rs *rowStreamer) buildSelect(st *binlogdatapb.MinimalTable) (string, error
 	buf.Myprintf(" from %v%s", sqlparser.NewIdentifierCS(rs.plan.Table.Name), indexHint)
 	if len(rs.lastpk) != 0 {
 		if len(rs.lastpk) != len(rs.pkColumns) {
-			return "", fmt.Errorf("primary key values don't match length: %v vs %v", rs.lastpk, rs.pkColumns)
+			return "", fmt.Errorf("cannot build a row streamer plan for the %s table as a lastpk value was provided and the number of primary key values within it (%v) does not match the number of primary key columns in the table (%d)",
+				st.Name, rs.lastpk, rs.pkColumns)
 		}
 		buf.WriteString(" where ")
 		prefix := ""