Add a check to rows.Err after processing all rows

Closes #822.

In https://github.com/go-sql-driver/mysql/issues/1075, @acharis notes
that the way the go-sql driver is written, query timeout errors don't
get set in `rows.Err()` until _after_ a call to `rows.Next()` is made.

Because this kind of error means there will be no rows in the result
set, the `for rows.Next()` will never enter the for loop, so we must
check the value of `rows.Err()` after the loop, and surface the error up
appropriately.

go/logic/applier.go

@@ -444,6 +444,9 @@ func (this *Applier) CalculateNextIterationRangeEndValues() (hasFurtherRange boo
 			}
 			hasFurtherRange = true
 		}
+		if err = rows.Err(); err != nil {
+			return hasFurtherRange, err
+		}
 		if hasFurtherRange {
 			this.migrationContext.MigrationIterationRangeMaxValues = iterationRangeMaxValues
 			return hasFurtherRange, nil