Support error code in record level in lead bulk api

[Gaoan_Guo]

Recently, while I was implementing features in my local, it had took me more than 30 minutes to upload a file with 3 records, also I found the marketo failures api doesn't work correctly, 
first it can't report meaningful message , I didn't find any information in  the wiki(https://developers.marketo.com/rest-api/error-codes/), the second bad thing is, from the response csv file, it can't provide the error code column,
we can never know the status code 
X@X-OptiPlex-7020:~$ curl 'https://091-CTR-291.mktorest.com/bulk/v1/leads/batch/566157/failures.json?access_token=#credential#'
 advertiserId,atsJobId,campaignName,jobTitle,Import Failure Reason 
230933980,2,test campaign2,test job title2,Unknown error 
230933980,1,test campaign1,test job title1,Unknown error 
230933980,1,test campaign3,test job title won't override 1,Unknown error

After I chatted with custom service, case number(01418355), confirmed the bulk api & json rest api response is not identical on response format in importing leads, 

bulk api doesn't have an record level error code which json rest api has.

To make the api identical and also beneficial for users debugging, handling errors, it's very helpful add same error code support for bulk api.

[Kenny_Elkington]

For a few technical reasons, it isn't feasible for use to reuse the standard set of REST API errors in the bulk import APIs.  However, if you observe Unknown Errors as shown in the example above, they should be reported to support so that we can update it with a useful error message.