Add an error bag to the Bus Batch

[ohnotnow]

We uses batches quite a bit - mostly for importing large spreadsheets. We usually allowFailures() and use some sort of redis store to record any errors which are then sent to the user in the finally() block.

But it's a bit of a faff to write each time and to test.

So I was thinking of adding an error bag to the batch so you could do something like $this->batch->addError('rows', 'Row 1234 missing shipped date (column F)') and then be able to pull those out in the finally/then/whatever callback. Also allowing the testing possibility of something like $this->assertBatchHasErrors('rows', 'Row 1234 missing.....').

Just not sure if I'm the only one having to do this by hand all the time.