Add number of error records and their line numbers to PreviewResult

[muga]

We should add more information to PreviewResult. Because we check the result of PreviewExecutor and often determine whether we should execute run command or not. The current PreviewResult provides schema and list of ignored exceptions for users. I think that it's better to add more:

• of records that PreviewExecutor read.

• of error records

• actual error raw records and those line number (optional)

I also think that the above information can be build by using CommitReport on https://github.com/embulk/embulk/blob/master/embulk-core/src/main/java/org/embulk/exec/PreviewExecutor.java#L106

[muga]

This ticket is related to ticket #46